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If A+B+C=(3pi)/(2), prove that cos ^(2)A...

If `A+B+C=(3pi)/(2)`, prove that `cos ^(2)A+ cos ^(2) B- cos ^(2)C=-2 cos A cos B sin C.`

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The correct Answer is:
`=-2 cos A cos B sin C =R.H.S`
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