Tích phân $I=\int\limits_{2}^{{2\sqrt{5}}}{{\frac{x}{{({{x}^{2}}+1)\sqrt{{{{x}^{2}}+5}}}}dx}}$ bằng? A. $\ln \frac{{15}}{7}.$ B. $\frac{1}{3}\ln \frac{5}{7}.$ C. $\frac{1}{2}\ln \frac{{15}}{7}.$ D. $\frac{1}{4}\ln \frac{{15}}{7}.$
Đáp án đúng: D Đặt $t=\sqrt{{{{x}^{2}}+5}}=>I=\int\limits_{3}^{5}{{\frac{{dt}}{{{{t}^{2}}-4}}=\int\limits_{3}^{5}{{\frac{{dt}}{{(t-2)(t+2)}}=\frac{1}{4}\ln \frac{5}{7}.}}}}$