x(1+y平方)dx-y(1+x平方)dy=0x(1+y平方)dx=y(1+x平方)dyy/(1+y²)dy=x/(1+x²)dx2y/(1+y²)dy=2x/(1+x²)dx1/(1+y²)d(1+y²)=1/(1+x²)d(1+x²)两边同时积分得ln((1+y²))=ln(...