From the given figure, prove that $[\triangle ABC]=[\triangle CDE]$ From the given figure, prove that $\triangle ABC=\triangle CDE$ in area. 

In $\triangle ABC$ and $\triangle CDE$
1. $AB=CD$
2. $BC=DE$
3.??

It is easy to see with a picture; simply stick the triangles together by a side that is the same length, and see (where the two green sides are the same and the two blue sides are the same):

It is now trivial that the area is equal, since their base and height are equal.
