把x=3代入原方程,得：
9-6m+2m²-4mn+4n²=0
(m²-6m+9)+(m²-4mn+4n²)=0
(m-3)²+(m-2n)²=0
m-3=0 且 m-2n=0
m=3, n=1.5
mn=3×1.5=4.5