若x+y=7,xy=12,则x^2-xy+y^2为和值

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若x+y=7,xy=12,则x^2-xy+y^2为和值若x+y=7,xy=12,则x^2-xy+y^2为和值若x+y=7,xy=12,则x^2-xy+y^2为和值x^2-xy+y^2＝x^2+2xy+

若x+y=7,xy=12,则x^2-xy+y^2为和值
若x+y=7,xy=12,则x^2-xy+y^2为和值

若x+y=7,xy=12,则x^2-xy+y^2为和值
x^2-xy+y^2
＝x^2+2xy+y^2-3xy
＝（x+y）^2-3xy
=7^2-3x12
=49-36
=13

x^2-xy+y^2
=（x+y)^2-3xy
=7*7-3*12
=49-36
=13

x^2-xy+y^2=x^2+y^2+2xy-3xy=(x+y)^_3xy=7^2-3*12=49-36=13

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