Question

已知x+y=7,且xy=9,则x-y

Answer 1

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