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If cos a + cos b = 0 = sin a + sin b then cos 2a + cos 2b = |
| Given, cos A + cos B = 0 ...(1) and sin A + sin B = 0 ...(2) Squaring and subtracting (2) from (1), we get  Thus, cos2A + cos 2B = –2 cos (A + B) |
