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Binomial Expansion question C4

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    Find the first three terms of the binomial expansion of:

    4(2x-1)^{-1} -2(x^{2}+1)^{-1}

    Here is what I did:

     4(-1)^{-1} (-2x+1)^{-1} 

=  -4(1 + 2x + 4x^2) = -4 -8x -16x^2



-2(x^2 +1)^{-1}

= -2(1 - x^2 + x^4) = -2 +2x^2 -2x^4



-2 + 2x^2 - 2x^4 -4 -8x -16x^2 = -6 - 8x - 14x^2 -2x^4

    So the first three terms are -6, -8x and -14x^2 but this doesn't seem to be correct. Where have i gone wrong in the method?
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    (Original post by IDontKnowReally)
    Find the first three terms of the binomial expansion of:

    4(2x-1)^{-1} -2(x^{2}+1)^{-1}

    Here is what I did:

     4(-1)^{-1} (-2x+1)^{-1}  

=  -4(1 + 2x + 4x^2) = -4 -8x -16x^2 

 

-2(x^2 +1)^{-1} 

= -2(1 - x^2 + x^4) = -2 +2x^2 -2x^4 

 

-2 + 2x^2 - 2x^4 -4 -8x -16x^2 = -6 - 8x - 14x^2 -2x^4

    So the first three terms are -6, -8x and -14x^2 but this doesn't seem to be correct. Where have i gone wrong in the method?
    It is correct.
 
 
 
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