X9

MITTENPUNKT

X9 is the symmedian point of the excentral triangle.
< A B C >: Reference Triangle △ABC < J_a J_b J_c >: Excentral Triangle △J_a J_b J_c < M_a M_b M_c >: Medial Triangle △M_a M_b M_c [ A B ]: Sideline AB (Base c = 1) [ B C ]: Sideline BC (Side a) [ C A ]: Sideline CA (Side b) [ J_a M_a ]: Mittenpunkt Symmedian J_a–M_a [ J_b M_b ]: Mittenpunkt Symmedian J_b–M_b [ J_c M_c ]: Mittenpunkt Symmedian J_c–M_c { 1, 6 }: Incenter–Symmedian Line { 2, 7 }: Centroid–Gergonne Line { 4, 10 }: Orthocenter–Spieker Line { 21, 41 }: Central Line L(21, 41) A = Vertex A (0.0000, 0.0000) B = Vertex B (1.0000, 0.0000) C = Vertex C (0.6122, 0.5999) J_a = A-Excenter J_a (1.2857, 0.5249) J_b = B-Excenter J_b (-0.2857, 0.6999) J_c = C-Excenter J_c (0.4286, -1.0498) M_a = Midpoint M_a of BC (0.8061, 0.2999) M_b = Midpoint M_b of CA (0.3061, 0.2999) M_c = Midpoint M_c of AB (0.5000, 0.0000) X_1 = INCENTER (0.5714, 0.2333) X_6 = SYMMEDIAN POINT (LEMOINE POINT, GREBE POINT) (0.6000, 0.2672) X_2 = CENTROID (0.5374, 0.2000) X_7 = GERGONNE POINT (0.5903, 0.2769) X_4 = ORTHOCENTER (0.6122, 0.3958) X_10 = SPIEKER CENTER (0.5204, 0.1833) X_21 = SCHIFFLER POINT (0.5287, 0.1771) X_41 = -CEVA CONJUGATE OF X(31) (0.5823, 0.2244) X_9 = MITTENPUNKT (0.5110, 0.1615) A B C Ja Jb Jc Ma Mb Mc X1 X6 X2 X7 X4 X10 X21 X41 X9

X9 (MITTENPUNKT) constructed on acute reference triangle \((a, b, c) = (5, 6, 7)\) with unit baseline AB = \(1\).

Constructed Elements

  • Derived Triangle: Excentral Triangle △JaJbJc — vertices Ja, Jb, Jc (Anticevian triangle of Incenter X1 (the three excenters))

  • Derived Triangle: Medial Triangle △MaMbMc — vertices Ma, Mb, Mc (Formed by the midpoints of sides BC, CA, AB)

  • Concurrence Lines: Ja–Ma, Jb–Mb, Jc–Mc concurring at X9

Trilinears: \(b + c - a : c + a - b : a + b - c\)
Barycentrics: \(a(b + c - a) : b(c + a - b) : c(a + b - c)\)
Tripolars: \(\sqrt{b c (a^{4}-2 a^{2} b^{2}+b^{4}-4 b^{3} c-2 a^{2} c^{2}+6 b^{2} c^{2}-4 b c^{3}+c^{4})} : :\)
Search Value (6, 9, 13): \(3.571972699230\)
▸ Alternate Coordinates8 forms
  • Trilinears: \(\cot(A/2) : \cot(B/2) : \cot(C/2)\)

  • Trilinears: \(a - s : b - s : c - s\)

  • Trilinears: \(\csc A + \cot A : :\)

  • Trilinears: \(\csc A (1 + \cos A) : :\)

  • Trilinears: \(\tan A' : :\), where JaJbJc = excentral triangle

  • Trilinears: \(d(a,b,c) : :\), where d(a,b,c) = distance from A to the Gergonne line

  • Trilinears: square of semi-minor axis of A-Soddy ellipse : :

  • Barycentrics: \(1 + \cos A : 1 + \cos B : 1 + \cos C\)

▸ Linear Combinations1 combination
  • X9 = \((r + 2R)\) X1 - \(6R\) X2 - \(2r\) X3 = \(3\) X2 - X7

X9 lies on the these conics: Feuerbach circumhyperbola, Feuerbach circumhyperbola of the medial triangle, Jerabek circumhyperbola of the excentral triangle, Mandart hyperbola

X9 lies on these curves: K002, K132, K202, K207, K220, K251, K294, K332, K343, K345, K351, K352, K363, K384, K387, K453, K637, K696, K697, K710, K716, K717, K760, K761, K817, K880, K950, K970, K977, K980, K982, K984, K1025, K1038, K1044, K1055, K1059, K1060, K1077, K1079, K1080, K1081, K1082, K1083, K1084, K1090, Q151

X9 lies on the curve Q151 and on conics and cubics listed below.

▸ Incident Central Lines1141 lines

{1, 6}, {2, 7}, {3, 84}, {4, 10}, {5, 1729}, {8, 346}, {11, 3254}, {12, 5857}, {20, 10429}, {21, 41}, {22, 5314}, {25, 5285}, {30, 3587}, {31, 612}, {32, 987}, {33, 212}, {34, 201}, {35, 90}, {36, 2178}, {38, 614}, {39, 978}, {42, 941}, {43, 256}, {46, 79}, {48, 101}, {51, 3690}, {55, 200}, {56, 1696}, {58, 975}, {65, 4047}, {69, 344}, {75, 190}, {77, 651}, {80, 528}, {81, 5287}, {85, 10509}, {86, 2279}, {87, 292}, {92, 6358}, {99, 35106}, {100, 1005}, {105, 4712}, {108, 3213}, {109, 2199}, {114, 24469}, {119, 2950}, {123, 20623}, {124, 5513}, {140, 5843}, {141, 4422}, {145, 4029}, {164, 168}, {165, 910}, {171, 1707}, {172, 5019}, {173, 177}, {182, 7193}, {184, 26885}, {192, 239}, {193, 3879}, {194, 16827}, {205, 2359}, {216, 13006}, {222, 17811}, {223, 1073}, {228, 1011}, {241, 269}, {244, 17125}, {255, 25063}, {257, 17743}, {258, 7028}, {259, 15997}, {261, 645}, {264, 1948}, {273, 26003}, {275, 26941}, {291, 17065}, {294, 1253}, {306, 5739}, {312, 314}, {318, 1896}, {319, 17233}, 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23640}, {19604, 25731}, {19785, 26723}, {19804, 32939}, {19872, 32632}, {20080, 29583}, {20092, 24184}, {20106, 26934}, {20174, 29773}, {20205, 21370}, {20247, 25261}, {20248, 26653}, {20257, 27304}, {20305, 34176}, {20370, 34832}, {20430, 37510}, {20444, 35550}, {20456, 22172}, {20486, 25613}, {20544, 24329}, {20608, 21250}, {20662, 21320}, {20667, 26069}, {20678, 23868}, {20719, 31785}, {20930, 28980}, {20973, 21858}, {20979, 21389}, {20980, 21348}, {20984, 22174}, {21010, 36635}, {21026, 31134}, {21066, 26793}, {21090, 24298}, {21281, 30030}, {21286, 26581}, {21319, 37319}, {21367, 32782}, {21368, 24611}, {21379, 33681}, {21405, 27390}, {21514, 37597}, {21759, 23566}, {21827, 23543}, {21832, 24121}, {21956, 32865}, {22003, 24435}, {22076, 37324}, {22758, 37611}, {23062, 23618}, {23073, 30389}, {23354, 25292}, {23529, 28118}, {23649, 28352}, {24067, 27368}, {24154, 24156}, {24155, 24157}, {24158, 24242}, {24172, 28090}, {24210, 33137}, {24268, 25252}, {24325, 32935}, {24343, 25107}, {24346, 36008}, {24398, 27918}, {24411, 34361}, {24512, 26102}, {24542, 33122}, {24547, 24612}, {24549, 33821}, {24586, 30758}, {24603, 28827}, {24633, 24993}, {24690, 30822}, {24935, 25669}, {25010, 30312}, {25057, 31171}, {25343, 30779}, {25457, 25682}, {25570, 25571}, {25660, 29456}, {25690, 25693}, {25716, 36628}, {25842, 25856}, {25850, 25858}, {25875, 34489}, {25885, 34036}, {25958, 29873}, {25960, 33119}, {25961, 33067}, {26006, 26668}, {26035, 31339}, {26068, 27020}, {26107, 26959}, {26592, 34388}, {26658, 34497}, {26724, 33146}, {26730, 26731}, {26756, 27073}, {26919, 26940}, {26933, 30739}, {27036, 27102}, {27044, 27136}, {27108, 27514}, {27253, 36854}, {27398, 37265}, {27507, 28789}, {28365, 37596}, {28604, 29576}, {29085, 36661}, {29381, 29536}, {29388, 29504}, {29395, 29423}, {29431, 29514}, {29570, 31313}, {29616, 32099}, {29632, 33065}, {29642, 33064}, {29643, 32843}, {29653, 32946}, {29664, 33107}, {29667, 33166}, {29674, 33082}, {29679, 33083}, {29681, 33153}, {29687, 33080}, {29711, 29716}, {29767, 30939}, {29826, 32944}, {29850, 32776}, {29851, 33069}, {29854, 32949}, {29855, 32775}, {29856, 34997}, {30295, 35990}, {30416, 30429}, {30417, 30430}, {30676, 30701}, {30695, 31994}, {30942, 34589}, {31316, 31343}, {31408, 31533}, {31534, 34495}, {31535, 34494}, {31547, 31565}, {31548, 31566}, {31937, 35239}, {32577, 33589}, {32771, 32938}, {32773, 33118}, {32778, 33164}, {32849, 33077}, {32860, 32936}, {32861, 33092}, {32862, 33075}, {32914, 32925}, {32947, 33117}, {33084, 33158}, {33096, 33111}, {33100, 33131}, {33101, 33130}, {33129, 33151}, {33132, 33154}, {33134, 33139}, {33700, 36906}, {33863, 37608}, {33934, 33951}, {34256, 34293}, {34926, 34932}, {35091, 35113}, {35128, 35129}, {35614, 35617}, {35659, 35668}, {36531, 36554}, {37234, 37585}, {37248, 37583}, {37507, 37609}

X9 lies on the following circumconics: Feuerbach circumhyperbola, Feuerbach circumhyperbola of the medial triangle, Jerabek circumhyperbola of the excentral triangle, Mandart hyperbola, and these: {{A, B, C,2,9,200,281,282,346,2184,2287,2297,4183,6605,7097,7110,14943,15889,15890,15891,15892,16016,19605,21446,23617,28071,28132,30705,31618,34525,36101,36627,36629,36796,36910,36916}} {{A, B, C,3,9,40,271,1167,1819,3342,3347,7013,7078,34902}} {{A, B, C,9,10,72,78,307,318,1793,3694,3710,3718,8806}} {{A, B, C,9,33,37,210,226,312,1826,1903,2250,2321,2341,8818,13455,25430,27475,35144,35354,36800}} {{A, B, C,9,41,213,1334,1400,2212,2279,2311,2333,18784,18785,35106}} {{A, B, C,9,63,219,268,1073,1260,1815,2322,2327,2328,2983,3692,7123,15629,36631}} {{A, B, C,9,75,518,522,765,1861,2751,3693,3717,9436,33676,36819}} {{A, B, C,9,87,238,242,261,1447,2726,3684,3685,3737,4076,7220,9282,9499,18786,36815}}

X9 lies on the inellipse through Xi for i = 9,57,604,2171,2289,3083,3084,6602,34544; the perspector of this ellipse is X4564.

X9 lies on the following cubics: K002, K132, K202, K207, K220, K251, K294, K332, K343, K345, K351, K352, K363, K384, K387, K453, K637, K696, K697, K710, K716, K717, K760, K761, K817, K880, K950, K970, K977, K980, K982, K984, K1025, K1038, K1044, K1055, K1059, K1060, K1077, K1079, K1080, K1081, K1082, K1083, K1084, K1090

▸ Functional Relations81 relations

Notes

Let A' be the orthocorrespondent of the A-excenter, and define B' and C' cyclically. The lines AA', BB', CC' concur in X9. (Randy Hutson, November 18, 2015)

Let HaHbHc be the orthic triangle. Let La be the antiorthic axis of AHbHc, and define Lb and Lc cyclically. Let A″ = Lb∩Lc. B″ = Lc∩La, C″ = La∩Lb. Triangle A″B″C″ is inversely similar to ABC, with similitude center X9. (Randy Hutson, November 18, 2015)

Let E be the locus of the trilinear pole of a line that passes through X1. The center of E is X9. Moreover, E passes through the points X100, X658, X662, X799, X1821, X2580, X2581 and the bicentric pairs PU(34), PU(75), PU(77), PU(79). Also, E is a circumellipse of ABC and an inellipse of the excentral triangle. (Randy Hutson, February 10, 2016)

The locus E also passes through the vertices of Gemini triangle 2. (Randy Hutson, November 30, 2018)

Let A' be the intersection of the tangents to the A-excircle at the intercepts with the circumcircle, and define B' and C' cyclically. The lines AA', BB', CC' concur in X9. (Randy Hutson, December 2, 2017)

Let A' be the perspector of the A-mixtilinear excircle, and define B' and C' cyclically. The lines AA', BB', CC' concur in X9. (Randy Hutson, December 2, 2017)

Let E be the circumellipse of T = ABC with center X9. Then ABC is a billiard orbit of E (3-periodic). If we fix E in the plane, all its triangular orbits (a set of “rotating” triangles T) have the same X9. Note that X9 is the point of concurrence of lines drawn from each excenter to the midpoint of the corresponding side of T. (Dan Reznik, June 30, 2019) See [1] Triangular Orbits in Elliptic Billiards: the Mittenpunkt X9 is stationary at the origin and [2] Triangular Orbits in Elliptic Billiards: the Mittenpunkt X9 is stationary at the origin.

NEW in 2020: A particularly fine article is recommended: ‘ Can the Ellliptic Billiard Still Surprise Us? ‘, by Dan Reznik, Ronaldo Garcia, and Jair Koiller , in Mathematical Intelligencer 42 (2020) 6-17. An online pdf is available: Click here.

Notes from Peter Moses (July 1, 2019) about the circumellipse E with center X9:

  • E passes through the vertices of these triangles: ABC; Honsberger (see X7670); Inner Conway (see X11677); Gemini 2, Gemini 30.

  • E passes through the point Xi for these i:

  • 88, 100, 162, 190, 651, 653, 655, 658, 660, 662, 673, 771, 799,

  • 823, 897, 1156, 1492, 1821, 2349, 2580, 2581, 3257, 4598, 4599

  • 4604, 4606, 4607, 8052, 20332, 23707, 24624, 27834, 32680.

  • The perspector of E is X1; the major axis of E passes through Xi for these i: 9, 2590, 3307, 24646, and the minor axis, for these i: 9, 2591, 3309, 24647. The ellipse E is the isogonal conjugate of the antiorthic axis.

  • Let g = length of semi-major axis of E; then g = Sqrt[(2 R (r + R + \(\sqrt{R(R - 2 r)}\)) s^2)/(r + 4 R)^2].

  • Let h = length of semi-minor axis of E: then h = Sqrt[(2 R (r + R - \(\sqrt{R(R - 2 r)}\)) s^2)/(r + 4 R)^2].

  • h/g = \(\sqrt{(OI - R) (OI + 3 R) / ((OI - 3 R) (OI + R))}\) = Sqrt[(r + R - \(\sqrt{R (R - 2 r)}\)) / (r + R + \(\sqrt{R (R - 2 r)}\))].

  • eccentricity of E: 2 \(\sqrt{OI R / ((3 R - OI) (OI + R))}\).

  • If F is a focus of E, then |FX9|^2 = 4 s^2 R \(\sqrt{R (R - 2 r)}\) / (r + 4 R)^2/.

  • The axes of E are the asymptotes of the Feuerbach hyperbola.

  • The area of E is π R S /(r (r + 4 R))^(3/2) = 4 π a b c / ( 2 b c + 2 c a + 2 a b - a^2 - b^2 - c^2 )^(3/2) * area(ABC).

The triangle tangent to the vertices of E is the excentral triangle.

Let A'B'C' be the intouch triangle of the anticomplementary triangle of ABC. The ellipse E passes through A', B', C'. See Three orbits in elliptic billiard. (Dan Reznik, July 1, 2019)

The circumellipse with center X9 meets the circumhyperbola with center X11 (i.e., the Feuerbach hyperbola) in X1156. (Dan Reznik, January 20, 2020.)

Each of the following cubics passes through the four foci (two real and two imaginary) of the ellipse E: K040, K351, K352, K710, K716, K1060. The two real foci are a pair of isogonal conjugates, and likewise for the two imaginary foci. Moreover, if \(p : q : r\) is on the circumcircle, then \(p/a : q/b : r/c\) is on E. (Peter Moses, July 2 and 3, 2019)

The ellipse E is the isogonal conjugate of the antiorthic axis, X44X513, with barycentric equation \(x + y + z = 0\), and E is the isotomic conjugate of the X514X661, with barycentric equation a x + b y + c z = 0. This line, denoted by L(31), is the perspectrix of the anticomplementary triangle and the inner Conway triangle (which is the intouch triangle of the anticomplementary triangle). Points lying on X514X661 include Xi for i = 514, 661, 693, 857, 908, 914, 1577, 1959, 2084, 2582, 2583, 3239, 3250, 3762, 3766, 3835, 3904, 3912, 3936, 3948, 4129, 4358, 4391, 4462, 4468, 4486, 4728, 4766, 4776, 4789, 4791, 4801, 4823, 4978, 5074, 5179, 6332, 6381, 6590, 8045, 14206, 14207, 14208, 14209, 14210, 14281, 14349, 14350, 14963, 18669, 18715, 24018, 30565, 30566, 30804, 30806, 32679. (Peter Moses, July 2 and 3, 2019)

For a dynamic graphic with many options, based on a triangle inscribed in the circumellipse with center X9, see Elliptic Billiard: Loci of Triangular Centers. (Dan Reznik, February 4, 2020)