If cos^2A+cos^2B+cos^2C=1,t h e n A B C ...

If `cos^2A+cos^2B+cos^2C=1,t h e n A B C` is (a)equilateral (b) isosceles (c)right angles (d) none of these

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`cos^2A+cos^2B+cos^2C = 1`
`=>cos^2A+cos^2B+-(1-cos^2C) = 0`
`=>cos^2A+cos^2B-sin^2C = 0`
`=>cos^2A+(cos(B+C)cos(B-C)) = 0`
`=>cos^2A+(cos(pi-A)cos(B-C)) = 0`
`=>cos^2A-cosAcos(B-C) = 0`
`=>cosA(cosA-cos(B-C)) = 0`
`=>cosA(cos(pi-(B+C))-cos(B-C)) = 0`
...

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