黎曼和參考樓上PDEMAN老師

\(\displaystyle u=\sqrt{1+4x^2}  \Rightarrow u^2=1+4x^2  \Rightarrow \frac{1}{4}udu=xdx\)
原積分式為\(\displaystyle \int_1^{\sqrt{5}} \frac{3}{4}du =\frac{3}{4}(\sqrt{5}-1)\)

謝謝PDEMAN老師,satsuki931000老師