Đáp án:
$\begin{array}{l}
23)\\
\int\limits_0^{15} {f\left( {5x} \right)dx} \\
= \dfrac{1}{5}\int\limits_0^{15} {f\left( {5x} \right).5dx} \\
= \dfrac{1}{5}.\int\limits_0^{15} {f\left( {5x} \right).d\left( {5x} \right)} \\
\left\{ {5x = u} \right. \Rightarrow \left\{ \begin{array}{l}
khi:5x = 0 \Rightarrow u = 0\\
khi;5x = 15 \Rightarrow u = 3
\end{array} \right.\\
\Rightarrow I = \dfrac{1}{5}\int\limits_0^3 {f\left( u \right)du} = \dfrac{1}{5}.10 = 2\\
\Rightarrow A\\
24)x = 2sint\\
\Rightarrow \left\{ \begin{array}{l}
dx = 2\cos tdt\\
\sqrt {4 - {x^2}} = 2\sqrt {1 - {{\sin }^2}t} = 2.\cos t\\
x = 0 \Rightarrow t = 0\\
x = 1 \Rightarrow t = \dfrac{\pi }{6}
\end{array} \right.\\
\Rightarrow I = \int\limits_0^{\dfrac{\pi }{6}} {\dfrac{1}{{2.\cos t}}.2{\mathop{\rm costdt}\nolimits} } \\
= \int\limits_0^{\dfrac{\pi }{6}} {dt} \\
\Rightarrow B
\end{array}$
