/sci/ - Science & Math

Given the simple condition triangle 1 similar to triangle 2 in an unknown trapezium (could be right angle one, could be equal-sided could be a wild fucking thing) how do you assert proportional side according to similarity rules. Is there some kind of technique/rule of thumb? Right now I just use my gut because "obviously" I can put AC:AC since other sides arent equal by definition (trapezium).

I understand it is
[math]
\frac{AB}{CD} = \frac{BC}{AC} = \frac{AC}{AD}
[/math]

but cant always rely on intuition, in some cases triangles can look VERY different because drawing always approximate.
I feel like I'm missing something?