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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