上海交通大学2019年数学分析真题
解:先进行变量变换:
$\begin{cases}
u &=x+y \\
v &=\frac{y}{x}
\end{cases}\Rightarrow \begin{cases}
x & =\frac{u}{1+v} \\
y & =\frac{uv}{1+v}.
\end{cases}$
$|J|=\begin{vmatrix}
\frac{1}{1+v} &-\frac{u}{(1+v)^2} \\
\frac{v}{1+v} &\frac{u}{(1+v)^2}
\end{vmatrix}=\frac{u}{(1+v)^2}.$
$D\rightarrow D':0\leq u\leq 1,0\leq v\leq 1.$
$\begin{align*}I&=\iint_D=\iint_{D'}\frac{u^2\ln(1+v)}{(1+v)^2\sqrt{1+u^2}} \\\\
&=\int_{0}^{1}\frac{\ln(1+v)}{(1+v)^2}dv\int_{0}^{1}\frac{1-(1-u^2)}{\sqrt{1-u^2}}du\\\\
&=(\frac{\pi}{4}+\frac{1}{2})\int_{0}^{1}\frac{\ln(1+v)}{(1+v)^2}dv\\\\
&=(\frac{\pi}{4}+\frac{1}{2})(\frac{1}{2}-\frac{1}{2}\ln2).
\end{align*}$