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Question

If xy+yz+zx=1, prove that x1x2+y1y2+z1z2=4xyz(1x2)(1y2)(1z2).

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Solution

Put x=tanA,y=tanB,z=tanC.
Then xy+yz+zx=1 gives
tanAtanB+tanBtanC+tanCtanA1=0
or S21=0 or 1S2=0
Now tan(A+B+C)=S1S31S2
=S1S30==tanπ2
A+B+C=π/2 or 2A+2B+2C=π
tan(2A+2B+2C)=tanπ=0
or S1S31S2=0S1=S3
or tan2A+tan2B+tan2C=tan2Atan2Btan2C.
or 2x1x2+2y1y2+2z1z2=8xyz(1x2)(1y2)(1z2)
or x1x2+y1y2+z1z2=4xyz(1x2)(1y2)(1z2).

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