In the binomial expansion of (a-b)n,n≥5, the sum of 5th and 6th terms is zero, then a/b equals

Given T5+T6=0⇒C4   nan-4 b4-C5   nan-5 b5=0⇒C4   nan-4 b4=C5   nan-5 b5⇒an-4 b4an-5 b5=C5   nC4   n⇒ab=n-5+15⇒ab=n-45