3.证明方程 dfrac (x)(y)dx=f(xy) 经变换 xy=u 可化为变量分离方程,并由此-|||-求解下列方程:-|||-(1) (1+(x)^2(y)^2)dx=xdy;-|||-(2) dfrac (x)(y)dx=dfrac (2+{x)^2(y)^2}(2-{x)^2(y)^2}