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 || 
,
draw 
|| (pr. 31.), meeting 
.
Draw 
.
Because || (const.)
= 
but =
∴ = , 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.
