(m+n)²+|m|=m
绝对值项和平方项恒非负,要等式成立,m≥0,等式变为
(m+n)²+m=m
(m+n)²=0
n=-m
|2m-n-2|=0
2m-n-2=0
2m-(-m)-2=0
3m-2=0
m=2/3
n=-2/3
mn=(2/3)(-2/3)=-4/9