m^2-mn=21 1mn-n^2=-12 21+2得m^2-mn+mn-n^2=m^2-n^=21-12=91-2得m^2-mn-(mn-n^2)=m^2-2mn+n^2=21-(-12)=33
m^2-n^2=21-12=9m^2-2mn+n^2=21-(-12)=33
