Commentary on Proposition XI.10

Back to Proposition XI.10.

[Folio 0296 | p. 292]

The result of this proposition does not appear to be quoted in Euclid until XII. 3; but Euclid no doubt inserted it here advisedly, because it has the effect of incidentally proving that the “inclination of two planes to one another,” as defined in XI. Def. 6, is one and the same angle at whatever point of the common section the plane angle measuring it is drawn.