Question

find the term containing x^7 in (x-5y)^13

Answer 1

the general term in the expansion is (-1)^r(13Cr)x(x^(13-r))(5y)^r
for x^7 13-r=7 or x=6
thus the 7th term contains x^7 and is (13C6)(5^6)(x^7)y^6