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expand ( 1 + x/2 - x to the power of 2 ) to the power of n in ascending powers of x until and including the term in x to the power of 3.

the answers given by the book is

1 + nx/2 + n(n-9)/8 x to the power of 2 + n(n-1)(n-26)/48 x to the power of 3 + ...

i just want to know whether the answer is correct or not.

if possible, please show me the working. tq.