已知m^2-mn=21、mn-n^2=-12求m^2-n^2、m^2-2mn+n^2

已知m^2-mn=21、mn-n^2=-12求m^2-n^2、m^2-2mn+n^2

解
m²；-mn=21
mn-n²；=-12
兩式相加
m²；-mn+mn-n²；=21+（-12）=9
∴
m²；-n²；=9
兩式相减
m²；-mn-nm+n²；=21-（-12）=21+12=33
∴m²；-2mn+n²；=33

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