1. ## Binomial Theorem problem

I'm a little on how the binomial theorem works. I have a problem where
Given that $(1+x^2)+(1+x^2)^2+...+(1+x^2)^{20}$,compute the coefficient of $x^2$.

I know in the binomial theorem I can set x to 1 and y to x to have
$(1+x)^n$ but do I have to do this 20 times to find when n=1,2,3 etc?

2. Originally Posted by guyonfire89
I'm a little on how the binomial theorem works. I have a problem where Given that $(1+x^2)+(1+x^2)^2+...+(1+x^2)^{20}$,compute the coefficient of $x^2$.
Here is a hint. The coefficient of $x^2$ in the expansion of $(1+x^2)^j$ is $j$.