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If A+B+C = 2S , then P.T cos(S-A)+cos...

If A+B+C = 2S , then
P.T `cos(S-A)+cos(S-B)+cos(S-C)+cosS=4cos.(A)/(2)cos.(B)/(2)cos.(C)/(2)`

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The correct Answer is:
`=4 cos. (A)/(2) cos. (B)/(2)cos . (C)/(2)= R.H.S`
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