11st.
(a-b)2cos2(c/2)+(a+b)2sin2(c/2)
=(a-b)2cos2(c/2)+(a+b)2-(a+b)2cos2(c/2)
=(a+b)2+cos2(c/2){(a-b)2-(a+b)2}
=a2+b2+2ab-4abcos2(c/2)
=a2+b2-2ab{2cos2(c/2)-1}
=a2+b2-2abcos c
=c2
4 Answers
213)
LHS = (b + c) cos A + (c + a) cos B + (a + b) cos C
LHS = [c cos B + b cos C] + a [a cos C + c cos A] + [b cos A + a cos B]
LHS = a + b + c = RHS
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