expand as a binomial series

Question

(r-p)^9

Answer 1

just look at row 10 of Pascal's Triangle, and use r and -p.

That is, (r-p)^9 = r^9 + 9r^8(-p) + 36r^7(-p)^2 + ...

= r^9 - 9r^8p + 36r^7p^2 - ...