Cho $\alpha=0$, ta được: $C\sin\beta=1$.
Cho $\alpha=\dfrac{\pi}{2}$, ta được: $C\sin\left(\beta+\dfrac{\pi}{2}\right)=1$.
Suy ra: $\sin\beta=\sin\left(\beta+\dfrac{\pi}{2}\right)$
$\Rightarrow \beta=\pi-\left(\beta+\dfrac{\pi}{2}\right)+k2\pi,k\in\mathbb{Z}$
$\Rightarrow \beta=\dfrac{\pi}{4}+k\pi,k\in\mathbb{Z}$.
Với $\beta=\dfrac{\pi}{4}+2k\pi,k\in\mathbb{Z}$, suy ra: $C=\sqrt2$
Với $\beta=\dfrac{\pi}{4}+(2k+1)\pi,k\in\mathbb{Z}$, suy ra: $C=-\sqrt2$