— GEOMETOR • elements

G.22 plane angle

A plane angle is the inclination to one another of two lines or arcs in a plane that meet and do not lie on the same line.

G.24 congruent triangles by SAS

Triangles with two sides and the included angle equal are congruent (SAS).

G.25 base angles of an isosceles triangle

In an isosceles triangle, the angles at the base are equal, and if the equal sides are extended, the angles under the base are equal.

G.27 triangle with equal base angles is isosceles

If two angles of a triangle are equal, the sides subtending the equal angles are also equal.

G.28 uniqueness of triangle sides on a base

On the same segment and on the same side, there cannot be constructed two distinct triangles with both pairs of conterminous sides equal.

G.29 congruent triangles by SSS

Triangles with all three corresponding sides equal have their corresponding angles equal (SSS).

G.30 rectilineal angle

A rectilineal angle is contained by two segments or lines.

G.33 right angle and perpendicular

When a segment standing on a line makes the adjacent angles equal, each of the equal angles is a right angle, and the standing segment is perpendicular to the line.

G.34 construct a perpendicular at a point on a line

Construct a segment at right angles to a given line from a given point on it.

G.35 construct a perpendicular from a point to a line

Construct a perpendicular to a given line from a given point not on it.

G.37 adjacent angles on a line sum to two right angles

A segment standing on a line makes either two right angles or adjacent angles summing to two right angles.

G.39 supplementary adjacent angles form a line

If two segments at a point on a line on opposite sides make adjacent angles summing to two right angles, the two segments are collinear.

G.40 vertical angles are equal

Intersecting lines make their vertical angles equal.

G.41 exterior angle of a triangle is greater than interior

In any triangle, if one side is extended, the exterior angle is greater than either opposite interior angle.

G.42 two angles of a triangle are less than two right angles

In any triangle, the sum of any two angles is less than two right angles.

G.43 greater side subtends greater angle

In any triangle, the greater side subtends the greater angle.

G.44 greater angle is subtended by greater side

In any triangle, the greater angle is subtended by the greater side.

G.45 triangle inequality

In any triangle, the sum of any two sides is greater than the remaining side.

G.46 interior segments of a triangle

Segments constructed from the endpoints of one side of a triangle to an interior point have a lesser sum but contain a greater angle than the other two sides.

G.48 construct an angle equal to a given angle

Construct on a given line at a given point an angle equal to a given angle.

G.49 hinge theorem

If two triangles have two sides equal to two sides respectively, the one with the greater included angle has the greater base.

G.50 converse hinge theorem

If two triangles have two sides equal to two sides respectively, the one with the greater base has the greater included angle.

G.51 congruent triangles by ASA and AAS

Triangles with two angles and one corresponding side equal are congruent (ASA / AAS).

G.53 equal alternate interior angles imply parallel lines

If a transversal cutting two lines makes the alternate interior angles equal, the lines are parallel.

G.54 equal corresponding angles imply parallel lines

If a transversal cutting two lines makes the exterior angle equal to the opposite interior angle, or interior angles on the same side supplementary, the lines are parallel.

G.55 parallel postulate

Two lines intersected by a transversal with interior angles on one side summing to less than two right angles meet on that side.

G.56 transversal angles of parallel lines

A transversal cutting parallel lines makes alternate interior angles equal, exterior and opposite interior angles equal, and same-side interior angles supplementary.

G.57 lines parallel to the same line are parallel

Lines parallel to the same line are parallel to one another.

G.58 construct a line parallel to a given line

Construct through a given point a line parallel to a given line.

G.59 exterior angle and angle sum of a triangle

In any triangle, an exterior angle equals the sum of the two opposite interior angles, and the three interior angles sum to two right angles.

G.60 segments joining equal and parallel segments

Segments joining the endpoints of equal and parallel segments in the same directions are themselves equal and parallel.

G.62 opposite sides, angles, and diagonal of a parallelogram

In a parallelogram, opposite sides and opposite angles are equal, and the diagonal bisects the area.

G.63 square, oblong, rhombus, and rhomboid

Of quadrilateral figures, a square is equilateral and right-angled; an oblong is right-angled but not equilateral; a rhombus is equilateral but not right-angled; a rhomboid has opposite sides and angles equal but is neither equilateral nor right-angled.

G.64 parallelograms on the same base in the same parallels

Parallelograms on the same base and in the same parallels are equal in area.

G.71 construct a parallelogram equal to a triangle

Construct, in a given angle, a parallelogram equal in area to a given triangle.

G.73 apply a parallelogram equal to a triangle on a segment

Apply to a given segment, in a given angle, a parallelogram equal in area to a given triangle.

G.74 construct a parallelogram equal to a polygon

Construct, in a given angle, a parallelogram equal in area to a given polygon.

G.75 construct a square on a segment

Construct a square on a given segment.

G.76 obtuse angle

An obtuse angle is greater than a right angle.

G.77 acute angle

An acute angle is less than a right angle.

G.78 right, obtuse, and acute-angled triangles

A right-angled triangle has a right angle, an obtuse-angled triangle has an obtuse angle, and an acute-angled triangle has three acute angles.

G.79 Pythagorean theorem

In a right-angled triangle, the square on the hypotenuse equals the sum of the squares on the two legs (Pythagorean Theorem).

G.80 converse of the Pythagorean theorem

If the square on one side of a triangle equals the sum of the squares on the other two sides, the angle contained by those two sides is right.

G.85 square of a sum

If a segment is cut at random, the square on the whole equals the squares on the parts together with twice the rectangle contained by the parts.

G.91 sum of squares on unequal parts

If a segment is cut into equal and unequal parts, the sum of the squares on the unequal parts is double the sum of the square on the half and the square on the segment between the points of section.

G.92 sum of squares on bisected and extended segment

If a segment is bisected and a segment is added to it in a line, the sum of the squares on the whole with the added segment and on the added segment is double the sum of the square on the half and the square on the half with the added segment.

G.93 cut a segment in extreme and mean ratio

Cut a given segment so that the rectangle contained by the whole and one of the segments equals the square on the remaining segment.

G.94 law of cosines for obtuse triangles

In obtuse-angled triangles, the square on the side subtending the obtuse angle is greater than the sum of the squares on the sides containing the obtuse angle by twice the rectangle contained by one of those sides and the projection of the other upon it.

G.95 law of cosines for acute triangles

In acute-angled triangles, the square on the side subtending an acute angle is less than the sum of the squares on the sides containing the acute angle by twice the rectangle contained by one of those sides and the projection of the other upon it.

G.98 find the center of a circle

Find the center of a given circle.

G.99 chord of a circle lies within the circle

The segment joining any two points on the arc of a circle falls within the circle.

G.100 diameter bisects chord iff perpendicular

A segment through the center of a circle bisects a chord not through the center if and only if it cuts the chord at right angles.

G.101 intersecting chords not through the center do not bisect

In a circle, two intersecting chords that do not both pass through the center do not bisect one another.

G.105 distances to a circle from an eccentric diameter point

From an eccentric point on the diameter of a circle, the greatest segment to the arc passes through the center, the least is the remainder of the diameter, and segments nearer the center are greater than those more remote, with at most two equal segments on opposite sides.

G.106 distances to a circle from an exterior point

From a point outside a circle, of segments falling on the concave arc the greatest passes through the center and nearer ones are greater, while on the convex arc the least lies between the point and the diameter and nearer ones are less, with at most two equal segments.

G.107 point with three equal segments to a circle is the center

A point within a circle from which more than two equal segments fall on the circle is the center of the circle.

G.114 equal chords are equidistant from the center

In a circle, chords are equal if and only if they are equally distant from the center.

G.116 diameter is the greatest chord

Of chords in a circle, the diameter is the greatest, and a chord nearer the center is always greater than one more remote.

G.118 perpendicular at extremity of a diameter is tangent

The line drawn at right angles to the diameter of a circle from its extremity falls outside the circle, no other line can be interposed between it and the arc, and the angle of the semicircle is greater than any acute rectilineal angle.

G.119 draw a tangent to a circle from a point

From a given point, construct a line tangent to a given circle.

G.120 radius to point of contact is perpendicular to tangent

If a line touches a circle, the segment joined from the center to the point of contact is perpendicular to the tangent.

G.121 perpendicular to tangent at contact passes through center

If a line touches a circle, the line drawn at right angles to the tangent from the point of contact passes through the center of the circle.

G.122 sector of a circle

A:term:**sector of a circle** <sector-of-circle> is the:term:figure <G.9> which, when an:term:angle <G.22> is constructed at the:term:center <center> of the:term:circle <G.10>, is contained by the:term:segments <G.4> containing the angle and the arc cut off by them.

G.123 inscribed angle theorem

In a circle, the angle at the center is double the angle at the circumference when the angles have the same arc as base.

G.124 angles in the same circle segment are equal

In a circle, angles in the same segment are equal to one another.

G.125 opposite angles of a cyclic quadrilateral are supplementary

The opposite angles of any quadrilateral inscribed in a circle sum to two right angles.

G.127 similar circle segments

Similar segments of circles are those which admit equal:term:angles <G.22>, or in which the angles are equal.

G.128 no two similar unequal circle segments on the same chord

On the same segment and on the same side of it, there cannot be constructed two similar and unequal segments of circles.

G.130 complete a circle from a given segment

Given a segment of a circle, complete the full circle of which it is a segment.

G.132 angle standing on an arc

And, when the:term:segments <G.4> containing the:term:angle <G.22> cut off a arc, the angle is said to:term:**stand upon** <stand-upon-arc> that arc.

G.133 equal angles stand on equal arcs

In equal circles, equal angles stand on equal arcs, whether they stand at the centers or at the circumferences.

G.134 angles standing on equal arcs are equal

In equal circles, angles standing on equal arcs are equal to one another, whether they stand at the centers or at the circumferences.

G.135 equal chords cut off equal arcs

In equal circles, equal chords cut off equal arcs, the greater equal to the greater and the less to the less.

G.136 equal arcs are subtended by equal chords

In equal circles, equal arcs are subtended by equal chords.

G.137 bisect an arc

Bisect a given arc of a circle.

G.138 angle in a circle segment

An:term:**angle in a segment** <angle-in-segment> is the:term:angle <G.22> which, when a:term:point <G.1> is taken on the arc of the segment and:term:segments <G.4> are joined from it to the:term:extremities <G.6> of the segment which is the:term:**base of the segment** <base-of-segment>, is contained by the segments so joined.

G.139 Thales’ theorem and angles in circle segments

In a circle, the angle in a semicircle is right, the angle in a greater segment is acute, and the angle in a lesser segment is obtuse.

G.140 alternate segment theorem

If a line touches a circle and a chord is drawn from the point of contact, the angles which the chord makes with the tangent equal the angles in the alternate segments of the circle.

G.141 construct a circle segment on a chord admitting a given angle

On a given segment, construct a segment of a circle admitting an angle equal to a given rectilineal angle.

G.142 cut off a circle segment admitting a given angle

From a given circle, cut off a segment admitting an angle equal to a given rectilineal angle.

G.143 intersecting chords theorem

If two chords in a circle intersect one another, the rectangle contained by the segments of one equals the rectangle contained by the segments of the other.

G.144 tangent-secant theorem

If from a point outside a circle a secant and a tangent are drawn, the rectangle contained by the whole secant and its external segment equals the square on the tangent.

G.145 converse of the tangent-secant theorem

If from a point outside a circle a secant and a segment to the circle are drawn such that the rectangle on the whole secant and its external part equals the square on the segment, the segment is tangent to the circle.

G.148 polygon inscribed in a circle

A:term:polygon <G.12> is said to be:term:**inscribed in a circle** <inscribed-in-circle> when each:term:angle <G.22> of the inscribed:term:figure <G.9> lies on the arc of the:term:circle <G.10>.

G.149 inscribe a triangle in a circle equiangular with a triangle

Inscribe in a given circle a triangle equiangular with a given triangle.

G.151 circumscribe a triangle about a circle equiangular with a triangle

Circumscribe about a given circle a triangle equiangular with a given triangle.

G.152 polygon inscribed in a polygon

A:term:polygon <G.12> is said to be:term:**inscribed in a polygon** <inscribed-in-rectilineal-figure> when the respective:term:angles <G.22> of the inscribed:term:figure <G.9> lie on the respective sides of that in which it is inscribed.

G.154 inscribe a circle in a triangle

Inscribe a circle in a given triangle.

G.155 polygon circumscribed about a polygon

Similarly a:term:figure <G.9> is said to be:term:**circumscribed about a figure** <circumscribed-about-figure> when the respective sides of the circumscribed figure pass through the respective:term:angles <G.22> of that about which it is circumscribed.

G.156 circle circumscribed about a polygon

A:term:circle <G.10> is said to be:term:**circumscribed about a figure** <G.155> when the arc of the circle passes through each:term:angle <G.22> of the:term:figure <G.9> about which it is circumscribed.

G.157 circumscribe a circle about a triangle

Circumscribe a circle about a given triangle.

G.158 inscribe a square in a circle

Inscribe a square in a given circle.

G.159 circumscribe a square about a circle

Circumscribe a square about a given circle.

G.160 inscribe a circle in a square

Inscribe a circle in a given square.

G.161 circumscribe a circle about a square

Circumscribe a circle about a given square.

G.162 construct a golden isosceles triangle

Construct an isosceles triangle having each of the angles at the base double the remaining angle.

G.163 inscribe a regular pentagon in a circle

Inscribe an equilateral and equiangular pentagon in a given circle.

G.164 circumscribe a regular pentagon about a circle

Circumscribe an equilateral and equiangular pentagon about a given circle.

G.165 inscribe a circle in a regular pentagon

Inscribe a circle in a given equilateral and equiangular pentagon.

G.166 circumscribe a circle about a regular pentagon

Circumscribe a circle about a given equilateral and equiangular pentagon.

G.167 inscribe a regular hexagon in a circle

Inscribe an equilateral and equiangular hexagon in a given circle.

G.212 angle triangle bisected

If an angle of a triangle be bisected and the segment cutting the angle cut the base also, the segments of the base will have the same ratio as the remaining sides of the triangle; and, if the segments of the base have the same ratio as the remaining sides of the triangle, the segment joined from the vertex to the point of section will bisect the angle of the triangle.

G.213 equiangular triangles sides

In equiangular triangles the sides about the equal angles are proportional, and those are corresponding sides which subtend the equal angles.

G.214 triangles sides proportional

If two triangles have their sides proportional, the triangles is equiangular and will have those angles equal which the corresponding sides subtend.

G.215 triangles angle equal

If two triangles have one angle equal to one angle and the sides about the equal angles proportional, the triangles is equiangular and will have those angles equal which the corresponding sides subtend.

G.216 triangles angle equal

If two triangles have one angle equal to one angle, the sides about other angles proportional, and the remaining angles either both less or both not less than a right angle, the triangles is equiangular and will have those angles equal, the sides about which are proportional.

G.217 similar rectilineal figures

Similar polygons are such as have their:term:angles <G.22> severally equal and the sides about the equal angles:term:proportional <G.180>.

G.218 right angled triangle

If in a right-angled triangle a perpendicular be drawn from the right angle to the base, the triangles adjoining the perpendicular are similar both to the whole and to one another.

G.219 cut nth part

From a given segment to cut off a prescribed part.

G.220 cut similarly

Cut a given uncut segment similarly to a given cut segment.

G.221 third proportional

To two given segments to find a third proportional.

G.222 fourth proportional

To three given segments to find a fourth proportional.

G.223 mean proportional

To two given segments to find a mean proportional.

G.225 equal equiangular parallelograms

In equal and equiangular parallelograms the sides about the equal angles are reciprocally proportional; and equiangular parallelograms in which the sides about the equal angles are reciprocally proportional are equal.

G.226 equal triangles angle

In equal triangles which have one angle equal to one angle the sides about the equal angles are reciprocally proportional; and those triangles which have one angle equal to one angle, and in which the sides about the equal angles are reciprocally proportional, are equal.

G.227 segments proportional rectangle

If four segments be proportional, the rectangle contained by the extremes is equal to the rectangle contained by the means; and, if the rectangle contained by the extremes equals the rectangle contained by the means, the four segments is proportional.

G.229 describe on a given segment a rectilinear figure

Describe on a given segment a rectilinear figure similar and similarly situated to a given rectilinear figure.

G.231 similar triangles duplicate

Similar triangles are to one another in the duplicate ratio of the corresponding sides.

G.232 similar polygons divided

Similar polygons are divided into similar triangles, and into triangles equal in multitude and in the same ratio as the wholes, and the polygon has to the polygon a ratio duplicate of that which the corresponding side has to the corresponding side.

G.233 figures similar polygon

Figures which are similar to the same polygon are also similar to one another.

G.235 equiangular parallelograms ratio

Equiangular parallelograms have to one another the ratio compounded of the ratios of their sides.

G.236 parallelogram parallelograms diameter

In any parallelogram the parallelograms about the diameter are similar both to the whole and to one another.

G.237 construct one and the same figure similar to

Construct one and the same figure similar to a given polygon and equal to another given polygon.

G.238 parallelogram there taken

If from a parallelogram there be taken away a parallelogram similar and similarly situated to the whole and having a common angle with it, it is about the same diameter with the whole.

G.243 extreme and mean ratio

Cut a given segment in extreme and mean ratio.

G.244 right angled triangles

In right-angled triangles the figure on the side subtending the right angle is equal to the similar and similarly described figures on the sides containing the right angle.

G.245 triangles sides proportional

If two triangles having two sides proportional to two sides be placed together at one angle so that their corresponding sides are also parallel, the remaining sides of the triangles is in a segment.

G.246 equal circles angles

In equal circles angles have the same ratio as the arcs on which they stand, whether they stand at the centres or at the arcs.

G.385 magnitudes commensurable incommensurable

If two magnitudes be commensurable, and the one of them be incommensurable with any magnitude, the remaining one is incommensurable with the same.

G.396 square medial segment

The square on a medial segment, if applied to a rational segment, produces as breadth a segment rational and incommensurable in length with that to which it is applied.

G.477 area contained rational

If an area be contained by a rational segment and a first apotome, the “side” of the area is an apotome.

G.478 area contained rational

If an area be contained by a rational segment and a second apotome, the “side” of the area is a first apotome of a medial segment.

G.479 area contained rational

If an area be contained by a rational segment and a third apotome, the “side” of the area is a second apotome of a medial segment.

G.480 area contained rational

If an area be contained by a rational segment and a fourth apotome, the “side” of the area is minor.

G.481 area contained rational

If an area be contained by a rational segment and a fifth apotome, the “side” of the area is a segment which produces with a rational area a medial whole.

G.482 area contained rational

If an area be contained by a rational segment and a sixth apotome, the “side” of the area is a segment which produces with a medial area a medial whole.

G.506 segment set right

If a segment be set up at right angles to two segments which cut one another, at their common point of section, it is at right angles to the plane through them.

G.507 segment set right

If a segment be set up at right angles to three segments which meet one another, at their common point of section, the three segments are in one plane.

G.508 segments right angles

If two segments be at right angles to the same plane, the segments is parallel.

G.510 segments parallel right

If two segments be parallel, and one of them be at right angles to any plane, the remaining one is at right angles to the same plane.

G.512 segments meeting parallel

If two segments meeting one another be parallel to two segments meeting one another not in the same plane, they will contain equal angles.

G.515 point segments set

From the same point two segments cannot be set up at right angles to the same plane on the same side.

G.517 planes segment right

Planes to which the same segment is at right angles is parallel.

G.518 segments meeting parallel

If two segments meeting one another be parallel to two segments meeting one another, not being in the same plane, the planes through them are parallel.

G.522 segment right angles

If a segment be at right angles to any plane, all the planes through it is at right angles to the same plane.

G.524 solid angle

A:term:**solid angle** <solid-angle> is the inclination constituted by more than two:term:lines <G.2> which meet one another and are not in the same:term:surface <G.533>, towards all the lines. Otherwise: A:term:**solid angle** <solid-angle> is that which is contained by more than two:term:plane angles <G.22> which are not in the same:term:plane <G.7> and are constructed to one:term:point <G.1>.

G.525 solid angle contained

If a solid angle be contained by three plane angles, any two, taken together in any manner, are greater than the remaining one.

G.526 solid angle contained

Any solid angle is contained by plane angles less than four right angles.

G.527 there plane angles

If there be three plane angles of which two, taken together in any manner, are greater than the remaining one, and they are contained by equal segments, it is possible to construct a triangle out of the segments joining the extremities of the equal segments.

G.528 construct a solid angle out of three plane

Construct a solid angle out of three plane angles two of which, taken together in any manner, are greater than the remaining one: thus the three angles must be less than four right angles.

G.530 solid contained parallel

If a solid be contained by parallel planes, the opposite planes in it are equal and parallelogrammic.

G.532 segment point construct

On a given segment, and at a given point on it, to construct a solid angle equal to a given solid angle.

G.536 describe on a given segment a parallelepipedal solid

Describe on a given segment a parallelepipedal solid similar and similarly situated to a given parallelepipedal solid.

G.541 parallelepipedal solids equal

Parallelepipedal solids which are on equal bases and of the same height are equal.

G.543 similar parallelepipedal solids

Similar parallelepipedal solids are to one another in the triplicate ratio of their corresponding sides.

G.545 there equal plane

If there be two equal plane angles, and on their vertices there be set up elevated segments containing equal angles with the original segments respectively, if on the elevated segments points be taken at random and perpendiculars be drawn from them to the planes in which the original angles are, and if from the points so arising in the planes segments be joined to the vertices of the original angles, they will contain, with the elevated segments, equal angles.

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