Saturday, 1 October 2016

trigonometry - If A+B+C=pi, prove: cos(B+2C)+cos(C+2A)+cos(A+2B)=14cosfracBC2;cosfracCA2;cosfracAB2




If A+B+C=π, prove that:
cos(B+2C)+cos(C+2A)+cos(A+2B)=14cosBC2cosCA2cosAB2





My Attempt:



Here, A+B+C=π



Now,
LHS=cos(B+2C)+cos(C+2A)+cos(A+2B)=cos(B+C+C)+cos(C+A+A)+cos(A+B+B)=cos(π(AC))+cos(π(BA))+cos(π(CB))=cos(AC)cos(BA)cos(CB)



Please help to continue from here.


Answer



Let CA=2x,BC=2y,AB=2z2(x+y+z)=0



F=cos2x+cos2y+cos2z=2cos(x+y)cos(xy)+2cos2z1



Now as cos(x+y)=cos(z)=cosz,




F=2coszcos(xy)+2coszcos(x+y)1
=2cosz{cos(x+y)+cos(xy)}1=2cosz{2cosxcosy}1=?


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