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Review important identitie
cosx cosy = 1/2 [cos(x+y) + cos(x−y)]

Sagot :

Crisforpidn
cos(x+y) = cosxcosy - sinxsiny;
cos(x-y) = cosxcosy + sinxsiny;
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=> cos(x+y) + cos(x-y) = 2cosxcosy =>cosx cosy = 1/2 [cos(x+y) + cos(x−y)];
[tex]cosxcosy=\frac{1}{2}\left[cos(x+y)+cos(x-y)\right]\\\\R=\frac{1}{2}\left(cosxcosy-sinxsiny+cosxcosy-sinxsiny\right)\\\\=\frac{1}{2}\cdot2cosxcosy=cosxcosy=L\\\\R=L[/tex]
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