Câu 26.
Ta có: \(x_{12}=x_1+x_2;x_{23}=x_2+x_3;x_{13}=x_1+x_3\)
\(\Rightarrow\left\{{}\begin{matrix}x_1+x_2=6cos\left(\pi t+\dfrac{\pi}{6}\right)\\x_2+x_3=6cos\left(\pi t+\dfrac{2\pi}{3}\right)\\x_1+x_3=6\sqrt{2}cos\left(\pi t+\dfrac{\pi}{4}\right)\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x_1-x_3=6cos\left(\pi t-\dfrac{\pi}{12}\right)\\x_1+x_3=6\sqrt{2}cos\left(\pi t+\dfrac{\pi}{4}\right)\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}x_1=3\sqrt{6}cos\left(\pi t+\dfrac{\pi}{12}\right)\\x_3=3\sqrt{2}cos\left(\pi t+\dfrac{7\pi}{12}\right)\end{matrix}\right.\)
Nhận thấy \(\Delta\varphi=\varphi_2-\varphi_1=\dfrac{7\pi}{12}-\dfrac{\pi}{12}=\dfrac{\pi}{2}\Rightarrow\) hai dao động vuông pha.
Vậy \(x_1\) đạt cực đại thì \(x_3=0cm\)
Chọn A.
