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

x^2-xy+y^2
=x^2+2xy+y^2-3xy
=(x+y)^2-3xy
=7^2-3x12
=49-36
=13