Find the 5th term of the expression

Question

1) (a-b) ^9

Answer 1

for (a+b)^n
term(r+1) = C(n,r)(a^(n-r) b^r)

so for (a-b)^9
term(5) = C(9,4)(a^5)(-b)^4
= 126a^5 b^4

Answer 2

31