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Question

Prove the following:
sin1x+sin1y+sin1z=π Show that x1x2+y1y2+z1z2=xyz

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Solution

sin1x+sin1y+sin1z=πLet x=sina,y=sinb and z=sinca+b+c=πx1x2+y1y2+z1z2=sina1sin2a+sinb1sin2b+sinc1sin2c=sinacosa+sinbcosb+sinccosc=sin2a+sin2b+sin2c2=2sin2a+2b2cos2a2b2+sin2c2=2sin a+b cos ab sin2c2=2sin(πc)cos(ab)+2sinc cosc2=2sinc cos(ab)+2sinc cosc2=sinc[cos(ab)+cosc]=sinc[2cos(ab+c2)cos(abc2)]=sinc[2cosπbb2cos(aπ+a2)]=sinc[2cos(π2b)cos(aπ2)]=sinc×2×sinb×sina=2sina sinb sinc=2xyz

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