[m]cos\frac{2x^3}{3}=1-\frac{(\frac{2x^3}{3})^2}{2!}+\frac{(\frac{2x^3}{3})^4}{4!}+...+(-1)^{n}\frac{(\frac{2x^3}{3})^{2n}}{(2n)!}+...[/m]
Упрощаем:
[m]cos\frac{2x^3}{3}=1-\frac{2^2\cdot x^6}{3^2\cdot 2!}+\frac{2^4\cdot x^{12}}{3^4\cdot 4!}+...+(-1)^{n}\frac{2^{2n}x^{6n}}{3^{2n}\cdot (2n)!}+...[/m]