In the given fig.,  and ∠PST = ∠PRQ. Prove that ∆PQR is an isosceles triangle.
In the given fig, ∆ABD is a right triangle, right angled at A and AC ⊥ BD. Prove that AB2 = BC. BD.
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In ∆ABD and ∆AEC, we have
∠ADB = ∠AEC
[Each equal to 90°]
∠BAD = ∠EAC [Common]
So, by AA-criterion of similarity, we have
∆BDA ~ ∆CEA or ∆ADB ~ ∆AEC
Clearly, ∆CDB is not similar to ∆BEC, because they are not equiangular.