Đặt: \( t\, =\, \sqrt{x^{2} \, +\, 3} \, \Leftrightarrow \, t^{2} \, =\, x^{2} \, +\, 3\, \Leftrightarrow \, t{\rm d}t=\, x{\rm d}x\)
Khi đó: \(\int \frac{x\, {\rm d}x}{\sqrt{x^{2} \, +\, 3} \, } \, =\, \int {\rm d}t\, = \, t+\, C\, =\, \sqrt{x^{2} \, +\, 3} \, +\, C.\)