(m²-n²)x²-4mnx=m²-n²（m²-n²≠0）
m²x²-n²x²-4mnx-m²+n²=0
m²x²-2mnx+n²-n²x²-2mnx-m²=0
(m²x²-2mnx+n)-(n²x²+2mnx+m²)=0
(mx-n)²-(nx+m)²=(mx-n+nx+m)(mx-n-nx-m)=0
mx-n+nx+m=0 或mx-n-nx-m=0即x=n-m/m+n 或x=n+m/m-n