$A \cos \varphi_0 = 2$  
$A \cos\left(\frac{2\pi}{3} + \varphi_0\right) = \frac{A}{2}$
$A \neq 0 \Rightarrow$ chia phương trình 2 cho $A$:  
$\cos\left(\frac{2\pi}{3} + \varphi_0\right) = \frac{1}{2}$
$\cos\left(\frac{2\pi}{3} + \varphi_0\right) = \cos\varphi_0 \cos\frac{2\pi}{3} - \sin\varphi_0 \sin\frac{2\pi}{3}$  
$\cos\frac{2\pi}{3} = -\frac{1}{2},\quad \sin\frac{2\pi}{3} = \frac{\sqrt{3}}{2}$
$\cos\left(\frac{2\pi}{3} + \varphi_0\right) = \left(-\frac{1}{2}\right)\cos\varphi_0 - \left(\frac{\sqrt{3}}{2}\right)\sin\varphi_0$
$\left(-\frac{1}{2}\right)\cos\varphi_0 - \left(\frac{\sqrt{3}}{2}\right)\sin\varphi_0 = \frac{1}{2}$
$\cos\varphi_0 = \frac{2}{A}$
$\left(-\frac{1}{2}\right)\left(\frac{2}{A}\right) - \left(\frac{\sqrt{3}}{2}\right)\sin\varphi_0 = \frac{1}{2}$
$-\frac{1}{A} - \left(\frac{\sqrt{3}}{2}\right)\sin\varphi_0 = \frac{1}{2}$
$\left(\frac{\sqrt{3}}{2}\right)\sin\varphi_0 = -\frac{1}{A} - \frac{1}{2}$
$\sin\varphi_0 = \left(\frac{2}{\sqrt{3}}\right)\left(-\frac{1}{A} - \frac{1}{2}\right)$
$\sin^2\varphi_0 + \cos^2\varphi_0 = 1$
$\left[\left(\frac{2}{\sqrt{3}}\right)\left(-\frac{1}{A} - \frac{1}{2}\right)\right]^2 + \left(\frac{2}{A}\right)^2 = 1$
$\frac{4}{3}\left(\frac{1}{A} + \frac{1}{2}\right)^2 + \frac{4}{A^2} = 1$
$\frac{4}{3}\left(\frac{2 + A}{2A}\right)^2 + \frac{4}{A^2} = 1$
$\frac{4}{3} \cdot \frac{(A+2)^2}{4A^2} + \frac{4}{A^2} = 1$
$\frac{(A+2)^2}{3A^2} + \frac{4}{A^2} = 1$
$(A+2)^2 + 12 = 3A^2$
$A^2 + 4A + 4 + 12 = 3A^2$
$2A^2 - 4A - 16 = 0$
$A^2 - 2A - 8 = 0$
$A = \frac{2 \pm \sqrt{36}}{2} = \frac{2 \pm 6}{2}$
$A = 4$ hoặc $A = -2$
$A = 4$
$\cos\varphi_0 = \frac{2}{4} = \frac{1}{2}$
$\varphi_0 = \pm \frac{\pi}{3}$