1. ## geometry

plz help...

Suppose we are given triangle ABC, and let D lies in (AB) be such that |angleDCA| = |angleABC|.
Prove that d(A, C) is the mean proportional between d(A, B) and d(A, D).

2. We have $\triangle ABC\sim\triangle ACD$
Then $\displaystyle\frac{AC}{AD}=\frac{AB}{AC}\Leftright arrow AC^2=AB\cdot AD$

3. thanks, but I don't understand how

triange ABC ~ triangle ACD

4. Originally Posted by jenjen
thanks, but I don't understand how

triange ABC ~ triangle ACD
the symbol ~ is a notation for similar, i.e. when he wrote $\Delta ABC \sim \Delta ACD$, then he meant that the two triangles are similar..

5. I understand that symbol, but I just don't know how they can be similar. Thanks

6. Ohhhhhh, I got it now. kool!