Question

计算(x+y)(x-3y)-my(nx-y)(m.n均为常数)的值.在把x,y的值代入计算时,粗心的小明和小亮都把y的值

Answer 1

原式=(x+y)(x-3y)-my(nx-y)=x^2-3xy+xy-3y^2-mnxy+my^2=x^2-(2+mn)xy+(m-3)y^2

当m-3=0,mn+2=0,即有m=3,n=-2/3时不论y取何值,结果只与X有关,与Y无关.

即有x^2=25,x=(+/-)5