Show that if a median of a triangle is one-half the side to which it is drawn, then it must be right.
If you don't know this circle theorem, then use the fact that you have two isosceles triangles - one on either side of the median. Then look at the angles in these triangles that are equal. Call the size of one pair , and the size of the other pair . Then (can you see why?). Therefore .
Can you fill in the details now?