$\int\limits_{}^{}\frac{x^2.dx}{\sqrt{x^2 - 2}} = \int\limits_{}^{}\frac{x.xdx}{\sqrt{x^2 -2}}$
đặt u = $\sqrt{x^2-2} \Leftrightarrow u^2 = x^2 -2 \Rightarrow udu =xdx $$\int\limits_{}^{}(\sqrt{u^2 +2} )du = \frac{u}{2}.\sqrt{u^2+2}+ \frac{2}{2}ln\left| {u+\sqrt{u^2+2}} \right| + C$