[TEX]x=tan t[/TEX]
[TEX]\righ \I:= \int_{0}^{\frac{\pi}{4}} ln
1+tantdt[/TEX]
[TEX]t= \frac{\pi}{4}-u[/TEX]
[TEX]\righ \I:= \int_{0}^{\frac{\pi}{4}} ln \frac{2}{tanu+1}du[/TEX]
[TEX]\righ 2\I:= \int_{0}^{\frac{\pi}{4}} ln2.du[/TEX]
[TEX]\righ I:= \frac{\pi}{8}ln2[/TEX]