已知m^2-mn=21,mn-n^2= -15,(1)则m^2-n^2=?(2)m^2-2mn+n^2=?(说出原因)

问题描述:

已知m^2-mn=21,mn-n^2= -15,(1)则m^2-n^2=?(2)m^2-2mn+n^2=?(说出原因)

m^2-n^2=m^2-mn+mn-n^2=21+(-15)=6
m^2-2mn+n^2=m^2-mn=21-(mn-n^2)= 21-(-15)=36