若x+y=7,xy=12,则x^2-xy+y^2为和值
问题描述：
若x+y=7,xy=12,则x^2-xy+y^2为和值
答
x^2-xy+y^2=x^2+y^2+2xy-3xy=(x+y)^_3xy=7^2-3*12=49-36=13
答
13
答
x^2-xy+y^2
＝x^2+2xy+y^2-3xy
＝（x+y）^2-3xy
=7^2-3x12
=49-36
=13
答
x^2-xy+y^2
=（x+y)^2-3xy
=7*7-3*12
=49-36
=13