Question -
Answer -
It is known that (r +1)th term, (Tr+1), in the binomialexpansion of (a + b)n is givenby 
.
Assuming that a5b7 occursin the (r + 1)th term of the expansion (a –2b)12, we obtain
Comparing the indices of a and b in a5 b7 andin Tr +1, we obtain
r = 7
Thus, the coefficient of a5b7 is 