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Question

If cos α+cos β=0=sin α+sin β, then prove that cos 2α+cos 2β=2 cos (α+β)

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Solution

cos α+cos β=0=sin α+sin β

Squaring on both sides gives

cos2 α+cos2 β+2 cos α cos β=sin2 α+sin2β+2 sin αsin β

Bring square terms on one side, we get

cos 2α+cos 2 β=2 (sin α sin β+cos α cos β)=2 cos (α+β)


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