Question

已知x+y=7且xy=12时,1/x-1/y的值等于?

Answer 1

1/x-1/y=(y-x)/xy=(y-x)/12

(x y)^2=49

x^2 2xy y^2=49

x^2 2xy y^2-4xy=1

x^2-2xy y^2=1

(x-y)^2=1

y-x=1

所以是1/12

但此处x,y等地位,我认为答案应该是正负1/12