Question

已知x+y=5,xy=2,求1/(1/x+1/y)的值

Answer 1

x-y/x=x/x-2y

(x-y)(x-2y)=x^2

x^2-2xy-xy+2y^2=x^2

2y^2=3xy

x/y=2/3

x的平方+y的平方=5,xy=2

x^2+2xy+y^2=5+2xy

(x+y)^2=9

x+y=3 或x+y=-3

1/1/x+1/y

=xy/(x+y)

=2/3 或-2/3