f(x) = x^3(x+x^3+\ldots +x^{19})(x^2 + x^4 + \ldots + x^{20})^2 \\ \\ =x^8(1+x^2+\ldots +x^{18})^3 \\ \\ = x^8\left(\displaystyle\frac{1-x^{20}}{1-x^2}\right)^3