You can put this solution on YOUR website! find the exact value of the expression cos 255 degrees.
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cos(x+y)=cosxcosy-sinxsiny
cos 255=cos(210+45)=(cos210*cos45-sin210*sin45)
cos 210º=cos 30º=-√3/2 in quadrant III where cos is negative
sin 210º=sin 30º=-1/2 in quadrant III where sin is also negative
cos(x+y)=(-√3/2*√2/2)-(-1/2*√2/2)
cos(x+y)=(-√6/4)-(-√2/4)
cos(x+y)=(-√6+√2)/4
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