Question

Am I dealing with harmonic stuff (trig)?

Answer 1

You just need to solve for when $y=0$:

$\dfrac{\cos8t-9\sin8t}4=0\implies\cos8t-9\sin8t=0$

$\implies\cos8t=9\sin8t$

$\implies\dfrac19=\dfrac{\sin8t}{\cos8t}=\tan8t$

$\implies8t=\tan^{-1}\dfrac19+n\pi$

$\implies t=\dfrac18\tan^{-1}\dfrac19+\dfrac{n\pi}8$

where $n$ is any integer. We only care about when $0\le t\le1$, which happens for $n\in\{0,1,2\}$.

$t=\dfrac18\tan^{-1}\dfrac19\approx0.01$

$t=\dfrac18\tan^{-1}\dfrac19+\dfrac\pi8\approx0.41$

$t=\dfrac18\tan^{-1}\dfrac19+\dfrac\pi4\approx0.80$