不是m2;-2mn+n2;么?
m2;-2mn+n2;=m2;-mn-(mn-n2)=21-(-12)=33代数式m² - 2mn - n²的值是多少？m^2-mn=m*(m-n)=21mn-n^2=n(m-n)=-12两式相除，m/n=21/(-12)=-7/4,m=-7/4n带入第2式得-7/4n^2-n^2=-12，n^2=48/11m² - 2mn - n²=m^2-mn-(mn-n^2)-2n^2=21-(-12)-2*48/11=33-96/11=247/11