In triangle ABC, angle bisectors AD and BE meet at a point I. Prove that angle DIB = 90 - angle BCA/2.
I'm purposely using a right-angled triangle because it gives me a better overall picture.
Angle ADB = 180º - (90º + 30º) = 60º
Angle DIB = 90º ∠BCA /2 ∠DIB = 90º - 30º/2 = 75º
Look at a triangle BID ∠DIB = 180º - (45º + 60º) = 75º
( This is one way to prove it...This may not be the answer they're looking for.)