### How to find all roots with solve?

I have the following quantity:

```
c_p = 1 - (4*sin(th)^2+2*5*sin(th)/pi+(5/(2*pi))^2)
```

and I am trying to find its roots (the solutions for `c_p==0`

).

When plotting `c_p`

as a function of `th`

between $-\pi$ and $\pi$, I can see the curve crosses the x-axis at four positions. However, `solve(c_p==0, th)`

is only giving two roots:
$$\left[{theta} = -\arcsin\left(\frac{5}{4 \ \pi} + \frac{1}{2}\right),\quad {theta} = -\arcsin\left(\frac{5}{4 \ \pi} - \frac{1}{2}\right)\right].$$
It appears that `solve`

can only find the roots that are in the domain of the $\arcsin$ function, i.e in the interval ~~$[-\pi, \pi]$. ~~$[-\pi/2, \pi/2]$. Is there a way to get the other two roots?