a-ab/a+b（a/a-b+b/b-a）=a-b+b(1-b+1-a)=a-b+2b-b^2-ab=a+b-ab+b^21-m-n/m+2n÷m²-n²/m²+4mn+4n²=1-m-n/m+2n/m^2-n^2/m^2+4mn+4n^2=(m^2-m^3-mn+2n-n^2+4m^3n+4n^2m^2)/m^2