确定题目没问题吗?(n-2n)^2=(-n)²=n²
(m-2n)^2 -(n-2n)^2
=m²-4mn+4n²-n²
=m²-4mn+3n²
法2：
(m-2n)^2 -(n-2n)^2
=(m-2n)² -n²
=(m-2n+n)(m-2n-n)
=(m-n)(m-3n)
=m²-4mn+3n²