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Find the coefficient of x in the expansi...

Find the coefficient of x in the expansion of `[sqrt(1+x^2) - x]^-1` in ascending power of x when `|x|<1`.

A

0

B

`1/2`

C

`-1/2`

D

`-1/8`

Text Solution

Verified by Experts

The correct Answer is:
D
`[sqrt(1+x^(2))-x]^(-1)=(1)/(sqrt(1+x^(2))-x) xx ((sqrt(1+x^(2))+x))/((sqrt(1+x^(2))+x))`
`= (sqrt(1+x^(2))+x)/(1+x^(2)-x^(2))=x+sqrt(1+x^(2))=x+(1+x^(2))^(1//2)`
`= x+1+1/2 x^(2) + 1/2(-1/2) (x^(4))/(2!) + "….."`
Therefore, the coefficient of `x^(4)` is `-1//8`.
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