1.

Using the fact that sin (A + B) = sin A cos B + cos A sin B and the differentiation, obtain the sum formula for cosines.

Answer»

sin (A + B) = sin A cos B + cos A sin B 

Diff : w:r.to B

cos(A + B) x \(\frac{dA}{dB}\)= (sinA x (- sinB) + cosA x cosB) x \(\frac{dA}{dB}\)

cos(A +b) = cosA cosB – sinA sinB 

Hence cosine formulae



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