SOLUTION: find the exact value of the expression cos 255 degrees

Question 482086: find the exact value of the expression
cos 255 degrees
Answer by lwsshak3(11628) (Show Source): 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 210�=cos 30�=-√3/2 in quadrant III where cos is negative
sin 210�=sin 30�=-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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