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For any triangle ABC, prove that`a(bcosC-ccosB)=b^2-c^2` |
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Answer» `a (b cos C - c cos B)` `=(b cos C + c cos B) (b cos C - c cos B)` `= b^(2) cos^(2) C - c^(2) cos^(2) B` `= b^(2)cos^(2) C -c^(2) B` `= b^(2) - c^(2) - (b^(2) sin^(2) C - c^(2) sin^(2) B)` `= b^(2) - c^(2)` [as by the sine rule, `b sin C = c sin B`] Alternativey, using cos B and cos C formulae, we can prove the result |
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