Tính:
1) $\int_{0}^{2}x^2\sqrt{4+x^2}.dx$
2) $\int_{\frac{1}{\sqrt{2}}}^{\frac{3}{\sqrt{2}}}\frac{\sqrt{9+2x^2}}{x^2}.dx$
2, $I=\int \dfrac{\sqrt{9+2x^2}}{x^2} dx= \int \dfrac{1}{x}.\sqrt{\dfrac{9}{x^2}+2} dx$
Đặt $\sqrt{\dfrac{9}{x^2}+2}=t \rightarrow \dfrac{9}{x^2}+2=t^2 \rightarrow x^2=\dfrac{9}{t^2-2}$
$\rightarrow 2tdt=\dfrac{-18}{x^3} dx \rightarrow \dfrac{dx}{x^3}=\dfrac{tdt}{-9}$
$\rightarrow I=-\int \dfrac{9}{t^2-2}.t.\dfrac{tdt}{9} =-\int \dfrac{t^2}{t^2-2}dt=-\int 1+\dfrac{1}{2\sqrt{2}(t-\sqrt{2})}-\dfrac{1}{2\sqrt{2}(t+\sqrt{2})} dt=....$