Evaluating $\sum_{n \geq 0} \frac{x^{8n}}{(8n)!}$ 
$$\sum_{n \geq 0} \frac{x^{8n}}{(8n)!}$$

Here's my try:
$$\sum_{n \geq 0, \text{even}} \frac{x^{4n}}{(4n)!}$$
$$=\sum_{n \geq 0} \frac{(-1)^n+1^n}{2} \frac{x^{4n}}{(4n)!}$$
By convergence I can split the sums.
$$=\frac{1}{2} \sum_{n \geq 0} \frac{x^{4n}}{(4n)!}+\frac{1}{2} \sum_{n \geq 0} (-1)^n \frac{x^{4n}}{(4n)!}$$
Now consider,
$$\sum_{n \geq 0} \frac{x^{4n}}{(4n)!}$$
$$=\sum_{n \geq 0, \text{even}} \frac{x^{2n}}{(2n)!}$$
$$=\sum_{n \geq 0} \frac{(-1)^n+1^n}{2} \frac{x^{2n}}{(2n)!}$$
$$=\frac{1}{2} \left( \cos(x)+\cosh (x) \right)$$
Now if I find,

$$\sum_{n \geq 0} (-1)^n \frac{x^{4n}}{(4n)!}$$

I'll be done with the problem. How do I do that?

Bonus question:

Compute
$$\sum_{n \geq 0} \frac{x^{3n}}{(3n)!}$$
For this one I'm out of ideas.
 A: If you allow complex numbers,
$$\sum_{n\ge0}(-1)^n\frac{x^{4n}}{(4n)!}=\sum_{n\ge0}\frac{(xe^{\pi i/4})^{4n}}{(4n)!}=\frac12(\cos(xe^{\pi i/4})+\cosh(xe^{\pi i/4}))$$
By Euler's formula and sum of angles formulas,
$$\Re(\cos(xe^{\pi i/4}))=\Re(\cosh(xe^{\pi i/4}))=\cos\left(\frac x{\sqrt2}\right)\cosh\left(\frac x{\sqrt2}\right)$$
Thus, we may conclude that
$$\sum_{n\ge0}(-1)^n\frac{x^{4n}}{(4n)!}=\cos\left(\frac x{\sqrt2}\right)\cosh\left(\frac x{\sqrt2}\right)$$

As a side note as kccu mentions, we have
$$f(x)=\sum_{n\ge0}\frac{x^{kn}}{(kn)!}\implies f^{(n)}(0)=\begin{cases}1&n\equiv0\mod8\\0&n\not\equiv0\mod8\end{cases}$$
which gives the basic differential equation
$$f(x)=f^{(k)}(x),f(0)=1,f'(0)=f''(0)=0=\dots=,f^{(k-1)}(0)=0$$
Thus, by auxiliary equations, we find that
$$x=x^k\implies x=0,e^{2\pi ni/(k-1)}$$
And then taking real parts and interpolating through the initial derivative values.
A: $\newcommand{\bbx}[1]{\,\bbox[8px,border:1px groove navy]{\displaystyle{#1}}\,}
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 \newcommand{\mc}[1]{\mathcal{#1}}
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\begin{align}
\sum_{n = 0}^{\infty}{x^{8n} \over \pars{8n}!} & =
\sum_{n = 0}^{\infty}{x^{n} \over n!}
{\sum_{k = 0}^{7}\bracks{\exp\pars{k\pi\ic/4}}^{n} \over 8} =
{1 \over 8}\sum_{k = 0}^{7}\sum_{n = 0}^{\infty}{\bracks{\exp\pars{k\pi\ic/4}x}^{n} \over n!}
\\[5mm] & =
\bbx{\ds{{1 \over 8}\sum_{k = 0}^{7}
\exp\pars{\exp\pars{k\,{\pi \over 4}\,\ic}x}}}
\end{align}
A: Here is an alternate explicit way to compute the series $\sum_0^{\infty} \frac{x^{pn}}{(pn)!}$ for any positive integer $p$.
Let $\zeta$ be a primitive $p^{th}$ root of unity. 
Fix some $n \not\equiv 0 \pmod p$. Then letting $x=\sum_{i=1}^p \zeta^{ni}$ we see that $\zeta^n x = x$ and $\zeta^n \neq 1$, so that $x=0$. When $n \equiv 0 \pmod p$, it is clear that $\sum_{i=1}^p \zeta^{ni}=p$.
Therefore $$\frac{1}{p}\big(e^x + e^{\zeta x} + e^{\zeta^2 x} +...+e^{\zeta^{p-1}x}\big) = \frac{1}{p} \sum_{n=0}^{\infty} (1+\zeta^n+...+\zeta^{(p-1)n})\frac{x^{n}}{n!} = \sum_{n=0}^{\infty} \frac{x^{pn}}{(pn)!}$$
