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Question
If A = 600 and B = 300, verify that:
cos (A + B) = cos A cos B - sin A sin B
Solution
cos (A + B) = cos 900 = 0
cos A cos B - sin A sin B = cos 600 cos 300 - sin 600 sin 300
=`(1/2 xx sqrt(3)/2 - sqrt(3)/2 xx1/2) = ( sqrt(3)/4 - sqrt(3)/4) = 0`
∴ cos (A + B) = cos A cos B - sin A sin B
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