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Question

If (1+X+X2)n=2nr=0arxr then, a12a2+3a3....2na2n is equal to

A
n
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B
n
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C
0
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D
2n
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Solution

The correct option is B n
(1+x+x2)n=a0+a1x1+a2x2...+a2nx2n
Differentiating with respect to x
n(1+x+x2)n1(2x+1)=a1+2a2x1...+2na2nx2n1
Substituting x=1 we get
n(11+1)n1(2+1)=a12a2+3a34a4...+(1)2n12na2nx2n1
=(n)(1)
=n

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