In order to prove angle it is enough to prove that .
Given, .....(1)
Since, so applying Pythagoras theorem in the right angled triangle ABC, we get
.....(2)
From (1) and (2), we get
Therefore, angle . (Converse of pythagoras theorem)
In a quadrilateral ABCD, ∠B=90∘,AD2=AB2+BC2+CD2, prove that ∠ACD=90∘