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

If `cos^2A+cos^2B+cos^2C=1, t h e n ABC` is

A

equilateral

B

isosceles

C

right angled

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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