Hi,

It looks like this is fromPrinceton review, right?

Actually C istrap answerhere.

Actually the opposite angles of equal sides of a triangle are equal.(mind it of aSingle triangle)

Here in the first case two sides are equal but that are fromdifferenttriangle(AD from ADB triangle, DC from BDC triangle); so that formula will not work here.

For the second case, both sides are from asingletriangle. So, the formula works here!!!!!!!