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Question

The positive value of λ for which the co-efficient of x2 in the expression x2(x+λx2)10 is 720, is :

A
5
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B
22
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C
3
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D
4
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Solution

The correct option is D 4
x2(x+λx2)10
x2{ 10Cr(x)(10r)2λrx2r}
x2[ 10Crλrx(105r)2]
coefficient of x2 means that x(105r)2 has to be x0
r=2
10C2λ2=720
λ2=16
λ=±4
positive value of λ is 4.

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