7 . ( a ) Using the extended binomial expansion , find the first three terms of the expansion of 1 � x
2 in ascending powers of x up to x , giving the 1� ax answer in terms of a .
[ 3 ]
It is given that the sum of the coefficient of where a � 0 .
( b ) ( i ) Find the value of a .
2 x and the constant term is 21 ,
( ii ) Hence , write down the coefficient of x . [ 3 ]
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