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| 1. |
How we can solve this (cos x + cos y) square + ( sin x + sin y ) square = 4cos square(x+y÷ 2) |
| Answer» LHS = cos2 x + cos2 y + 2 cos x cos y + sin2 x + sin2y + 2 sin x sin y = 2 + 2 ( cos x cos y + sin x siny ) = 2 ( 1 + cos ( x - y ) ) = 2 * 2 cos2( ( x - y ) / 2 ) (because 1.+ cos x=2cos2x / 2 ) = 4 cos2 ( (x - y ) / 2 ) = RHS\xa0 | |