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Question

If xy+yz+zx=1, then x1+x2+y1+y2+z1+z2 is equal to

A
2(1+x2)(1y2)(1z2)
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B
2(1x2)(1+y2)(1z2)
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C
2(1+x2)(1+y2)(1+z2)
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D
2(1x2)(1+y2)(1+z2)
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Solution

The correct option is D 2(1+x2)(1+y2)(1+z2)
Let x=tanA2 y=tanB2 and z=tanC2.
Where A+B+C=π.
Thence
tanA2.tanB2+tanB2.tanC2+tanC2tanA2=1 .... (property).
Now
x1+x2+y1+y2+z1+z2
=tan(A2)1+tan2(A2)+tan(B2)1+tan2(B2)+tan(C2)1+tan2(C2)
=12[sinA+sinB+sinC]
=12[4cosC2.cosB2.cosA2]
=2cosC2.cosB2.cosA2
=2(1+tanA2)2(1+tanB2)2(1+tanC2)2
=2(1+x2)(1+y2)(1+z2)

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