m²-n²
=(m²-mn)+(mn-n²)
=21+(-15)
=21-15
=6
m²-2mn+n²
=(m²-mn)-(mn-n²)
=21-(-15)
=21+15
=36求m²-n²与m²-n²-2mn+n²的值m²-n²-2mn+n² 题目有问题