BOOK I. PROP. XL. THEOR.
41
QUAL triangles (
and
) on equal bases, and on the same side, are between the same parallels.
If which joins the vertices of triangles be not ||
,
∴ =
, 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.