已知x+y=7,且xy=9,则x-y

问题描述:

已知x+y=7,且xy=9,则x-y

x+y=7,且xy=9,则
(x+y)^2=49
x^2+2xy+y^2=49
x^2+y^2=49-2xy=31
x^2-2xy+y^2=31-2xy=13
(x-y)^2=13
x-y=正负根号13