Is there a formula for the cosine of 1/5 of an angle? I'm trying to find a formula for $\cos \frac{x}{5} $ as follow.

By the elementary trigonometric identity, $\cos 5x = 16\cos^5 x - 20\cos^3 x + 5\cos x$

By putting $x=  \frac{x}{5}$ , and using the change of variable $ y = \cos \frac{x}{5} $, we get the quintic equation $ 16y^5 - 20y^3 + 5y - \cos x = 0$.

If we can solve this equation by radicals, then we can get a formula for $\cos \frac{x}{5} $

However, I can not see how we can resolve this equation. Is this a solvable quintic equation? If so, how can we solve it by radicals?
Depending on how you look at this situation, it is solvable by radicals. Let’s take your cosine, and as @mweiss does, let’s call it $C$. I should point out that the case $C=1$ is very special, since $16x^5-20x^3+5x-1=(x-1)(4x^2+2x-1)^2$, probably giving the Wolfram-generated roots that I (wrongly) disparaged in a comment.

In the general case, set $S$ to be the corresponding sine, $S=\sqrt{1-C^2}$, then $\zeta=C+iS$ is a point on the unit circle, and you’re talking about $\zeta^{1/5}$ and your answer is the real part of this. If you accept the fifth root of a real number as something understandable, I don’t think it can be justified to disparage the fifth root of a complex number.

To direct an abstract eye onto the problem, let’s take a general number $\kappa$, which will play the role of $C$, and look at the field $F_0=\Bbb Q(\kappa)$. Then we make a quadratic extension, adjoining $\sigma=\sqrt{1-\kappa^2}$, so that we have $F_1=F_0(\sigma)$, and then adjoin a fifth root of unity, $\omega_5$, where
$$
\omega_5=\frac{-1+\sqrt5}4+\frac i2\sqrt{\frac{5+\sqrt5}2}\,,
$$
and $F_2=F_1(\omega_5)$ is at worst quartic over $F_1$, with Galois group cyclic of order $4$, $2$, 0r $1$, depending on what $\kappa$ was.
Then adjoin $(\kappa+i\sigma)^{1/5}$ to get the final field $F_3$, and again this extension is Galois, with cyclic group of order $5$ or $1$. So, starting with $F_0=\Bbb Q(\kappa)$, we get a chain of radical extensions,
$$
F_0\subset F_1\subset F_2\subset F_3\,,
$$
each inclusion being a cyclic Galois extension. If you call $\xi=(\kappa+i\sigma)^{1/5}$, then its real part is $\frac12(\xi+1/\xi)$, the desired cosine.
p = 3, trisection
p &gt;= 3
Python explanation