# prove that cos theta cos theta/ 2 - cos 3 theta cos 9 theta/2 = sin 4 theta sin 7 theta/2 plzzzz

Dear student

$\mathrm{Note}:\phantom{\rule{0ex}{0ex}}\mathrm{sin}\left(-\mathrm{x}\right)=-\mathrm{sinx}\phantom{\rule{0ex}{0ex}}\mathrm{cosA}-\mathrm{cosB}=-2\mathrm{sin}\left(\frac{\mathrm{A}+\mathrm{B}}{2}\right)\mathrm{sin}\left(\frac{\mathrm{A}-\mathrm{B}}{2}\right)\phantom{\rule{0ex}{0ex}}2\mathrm{cosAcosB}=\mathrm{cos}\left(\mathrm{A}+\mathrm{B}\right)+\mathrm{cos}\left(\mathrm{A}-\mathrm{B}\right)$
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