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In triangleABC, prove that cos^(2)A+cos^...

In `triangleABC`, prove that `cos^(2)A+cos^(2)B+cos^(2)Cge(3)/(4)`.

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`cos^(2)A+cos^(2)B+cos^(2)C`
`=1-sin^(2)A+cos^(2)B+cos^(2)C`
`=1+cos(A-B)cos(A+B)+cos^(2)C`
`=1-cos(A-B)cosC+cos^(2)C`
`ge1-cosC+cos^(2)C` [As `cos(A-B)le1`]
`=(cos C-1//2)^(2)+3//4ge3//4`
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