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I most humbly beseech help for this question.

If $A+B+C=180$ degrees, then prove $$ \sin^2(A)+\sin^2(B)-\sin^2(C)=2\sin(A)\sin(B) \cos(C) $$ I am not sure what trig identity I should use to begin this problem.

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    try substitution C = 180-A-B to get formulae for $\cos(C)$ and $\sin^2(C)$ in terms of trig functions of $A$ and $B$2012-07-31
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    thanks for your post I will study them and use them to solve the problem.2012-07-31
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    Fernando Martinez appreciates all the post and the work that went into answering his question.2012-08-01

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