Question

Calculate the coefficient of x4y18 in the expansion of (x − y)22

Answer 1

Binomial theorem:

$(x-y)^n=\displaystyle\sum_{k=0}^n\binom nkx^k(-y)^{n-k}$

The term with $x^4y^{18}$ corresponds to $n=18+4=22$ and $k=4$ (or $k=18$, since $\dbinom nk=\dbinom n{n-k}$).

So, the coefficient of this term is

$\dbinom{22}4(-1)^{18}=\dbinom{22}4=\dfrac{22!}{4!(22-4)!}=7315$