40
BOOK I. PROP. XXXIX. THEOR.
QUAL triangles
and
on the same base (
) and on the same side of it, are between the same parallels.
If , which joins the vertices of the triangles, be not ||
, draw
||
meeting
.
∴ =
, a part equal to the whole, which is absurd.
∴
; and in the same manner it can be demonstrated, that no other line except
is ||
; ∴
||
.
Q. E. D.