Which series represents cos(x)\cos(x)cos(x) for all real xxx?
∑n=0∞(−1)nx2n(2n)!\sum_{n=0}^{\infty} \frac{(-1)^n x^{2n}}{(2n)!}∑n=0∞(2n)!(−1)nx2n
∑n=0∞(−1)nxn(n)!\sum_{n=0}^{\infty} \frac{(-1)^n x^{n}}{(n)!}∑n=0∞(n)!(−1)nxn
∑n=0∞x2n(2n)!\sum_{n=0}^{\infty} \frac{x^{2n}}{(2n)!}∑n=0∞(2n)!x2n
∑n=0∞(−1)nx2n+1(2n+1)!\sum_{n=0}^{\infty} (-1)^n \frac{x^{2n+1}}{(2n+1)!}∑n=0∞(−1)n(2n+1)!x2n+1