Đặt \(3x=u\Rightarrow x=\dfrac{u}{3}\Rightarrow dx=\dfrac{1}{3}du\)
\(\left\{{}\begin{matrix}x=0\Rightarrow u=0\\x=\dfrac{\pi}{2}\Rightarrow u=\dfrac{3\pi}{2}\end{matrix}\right.\)
\(H=\dfrac{1}{3}\int\limits^{\dfrac{3\pi}{2}}_0sinu.du=-\dfrac{1}{3}.cosu|^{\dfrac{3\pi}{2}}_0=\dfrac{1}{3}\)