In the given figure, DE|| AC and DF || AE.

Prove that:

DE || AC [Given]

And we know, By Basic Proportional Theorem

If a line is drawn parallel to the one side of a triangle to intersect the other two sides in distinct points, then the other two sides are divided in same ratio

…[1]

And DF || AE

By Basic Proportional Theorem,

[From [1]]

Hence, Proved