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YettaMines860 YettaMines860
  • 26-03-2014
  • Mathematics
Answered

Review important identitie

cosx cosy = 1/2 [cos(x+y) + cos(x−y)]

Verified Answer :

crisforp
crisforp crisforp
  • 26-03-2014

cos(x+y) = cosxcosy - sinxsiny;
cos(x-y) = cosxcosy + sinxsiny;
..................................................
=> cos(x+y) + cos(x-y) = 2cosxcosy =>cosx cosy = 1/2 [cos(x+y) + cos(x−y)];

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Anonymous Anonymous
  • 28-03-2014

\(cosxcosy=\frac{1}{2}\left[cos(x+y)+cos(x-y)\right]\\\\R=\frac{1}{2}\left(cosxcosy-sinxsiny+cosxcosy-sinxsiny\right)\\\\=\frac{1}{2}\cdot2cosxcosy=cosxcosy=L\\\\R=L\)

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