已知m^2-mn=21,mn-n^2=-15,求m^2-2mn+n^2的值
问题描述：
已知m^2-mn=21,mn-n^2=-15,求m^2-2mn+n^2的值
答
∵m^2-mn=21,mn-n^2=-15
∴ m^2-mn-（mn-n^2﹚=21-﹙-15﹚
∴m^2-2mn+n^2=36