Tìm nguyên hàm sau:
1. $\int x^{3}\sqrt{x^{2}+1}.dx$2. $\int \frac{dx}{x^{2}+x+1}$
2. $A=\int \frac{dx}{x^{2}+x+1}=\int \frac{d(x+\frac{1}{2})}{(x+\frac{1}{2})^{2}+\frac{3}{4}}$
Đặt $x+\frac{1}{2}=\sqrt{\frac{3}{4}} tan u$
Ta có $A=\int \frac{d(\sqrt{\frac{3}{4}}tan u)}{\frac{3}{4}(tan^2u+1)}=\int \frac{\sqrt{\frac{3}{4}}(tan^2u+1)du}{\frac{3}{4}(tan^2u+1)}=\frac{2}{\sqrt{3}}\int {du}=\frac{2}{\sqrt{3}}u+c=\frac{2}{\sqrt{3}} arctan(\frac{2}{\sqrt{3}}(x+\frac{1}{2})+c$
- bleuceiu yêu thích