Question

In triangle ABC, LA = 60°. Prove that BC2 =AB2 + AC2 -AB.AC

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Answer 1

Answer:

that in a ΔABC in which ∠B is obtuse and AD ⊥ BC. To prove: AC2 = AB2 + BC2 + 2 BC X BD Proof: In ΔADC, AC2 = AD2 + DC2 ( Pythagoras theorem) AC2 = AD2 + (DB + BC)2 AC2 = AD2 + DB2 + BC2 + 2DB X BC. AC2 = AB2 + BC2 + 2 DB X BC. [ ∴ AD2 + DB2 = AB2 ]