Why Are Both X- and Y-Intercepts Equal to 1/f in the Lens' Equation?

[jaejoon89]

For the Lens' equation

1/f = 1/u + 1/v

why can both the x- and y-intercepts be considered equal to 1/f?

 

[Dadface]

When u becomes infinite what does 1/u become?What happens when v becomes infinite?