I'm looking, too. I am a colleague. Let me give you some ideas to solve the problem: AD is the midline of the bottom of the isosceles triangle, the height of the BC side and the bisector of the BAC angle. I is the perpendicular IE of AB, E is the vertical foot, IF is perpendicular to AC, F is the vertical foot, IE=ID (the distance from the bisector of the angle to both sides of the angle is equal, that is, c=a, similarly, b=c, so A = B = C.

I hope to adopt.