Question #7d988

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Answer 1 · 2016-12-18T15:20:35

#LHS=cos^2 76 + cos^2 16 - cos76 cos 16 #

#=1/2(2cos^2 76 + 2cos^2 16 - 2cos76 cos 16) #

#=1/2(1+cos152 + 1+cos32- cos(76+16)*cos(76- 16) #

#=1/2(2+2cos((152+32)/2) cos((152-32)/2)- cos(76+16)-cos(76- 16) #

#=1/2(2+2cos92cos60- cos92-cos60) #

#=1/2(2+2cos92xx1/2- cos92-1/2) #

#=1/2(2+cancelcos92- cancelcos92-1/2) #

#=1/2(2-1/2)=1/2*3/2=3/4=RHS#