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cosx + cosy = 0 ......1sinx + siny = 0 ......2Squaring both the equations and subtracting 2 from 1....we get:-> cos²x - sin²x + cos²y - sin²y + 2(cosxcosy - sinxsiny) = 0.....-> cos 2x + cos 2x + 2cos(x+y) = 0-> cos 2x + cos 2y = -2cos(x+y)....ans.....
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