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Prove that cosA -cosB -cosC =1-4sin(A/2)...

Prove that `cosA -cosB -cosC =1-4sin(A/2)cos(B/2)cos(C/2)`,if A+B+C=`pi`

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  1. Prove that cosA -cosB -cosC =1-4sin(A/2)cos(B/2)cos(C/2),if A+B+C=pi

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