已知m²-mn=21,mn-n2=12,则代数式m²-2mn+n²的值为
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已知m²-mn=21,mn-n2=12,则代数式m²-2mn+n²的值为
已知m²-mn=21,mn-n2=12,则代数式m²-2mn+n²的值为
已知m²-mn=21,mn-n2=12,则代数式m²-2mn+n²的值为
m²-mn=21①
mn-n²=12
-(mn-n²)=n²-mn=-12②
由①+②得m²-2mn+n²=21-12=9
m²-mn=21 mn-n²=12
(m²-mn)-(mn-n²)=21-12
m²-2mn+n²=9