$l=20(cm)=0,2(m)$
$\to A=\alpha_o.l=0,02(m)$
Khi $\alpha=0,05(rad)\to x=\alpha.l=0,01(m)$
$\omega=\sqrt{\dfrac{g}{l}}=7(rad/s)$
Ta có: $\dfrac{x^2}{A^2}+\dfrac{v^2}{A^2\omega^2}=1$
$\to \dfrac{0,01^2}{0,02^2}+\dfrac{v^2}{0,02^2.7^2}=1$
$\to v=0,1212(m/s)$