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Question 2:
Find the coefficient of a5b7 in (a – 2b)12
It is known that (r + 1)th term, (Tr+1), in the binomial expansion of (a + b)n is given by 
.
Assuming that a5b7 occurs in the (r + 1)th term of the expansion (a – 2b)12, we obtain
Comparing the indices of a and b in a5 b7 and in Tr +1, we obtain
r = 7
Thus, the coefficient of a5b7 is 
