Q. 23.7( 53 Votes )

Find the coefficient of

a5b7 in (a – 2b)12 .

Answer :

The general term Tr+1 in the binomial expansion is given by Tr+1 = nCr an-r br

Here a =a , b = -2b & n =12


Putting values


Tr+1 = 12Cr a12-r (-2b)r……….1


To find a5


We equate a12-r =a5


r=7


Putting r = 7 in 1


T8 = 12C7 a5 (-2b)7



= -101376 a5 b7


Hence the coefficient of a5b7= -101376


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