1. ## Binomial Theorem

Hello,

I was hoping to get a little help here. I have an assignment due in a couple of weeks, and one question is:

"Use the Binomial Theorem to determine the coefficient of x^15 in the expansion of (1/3x - 9)^20. Express your answer as a fraction in the lowest terms."

I am hoping that someone can point me in the general direction of been able to answer this one. I have read over the material, but just can't seem to get it.

Any help would be great.

Thanks.

Sean

2. I think you mean the Newton's binomial, which states that $(x+y)^n=\sum_{k=0}^n {n \choose k} x^{n-k} y^k$. Where ${n \choose k}=\frac{n!}{k!\,(n-k)!}$ are the coefficients. Just use this formula...