Real analysis - differential/sequence

[foxofdesert]

Homework Statement

I attached a .bmp file

Homework Equations

The Attempt at a Solution

I don't get this one at all. Since f is differentiable, the interval [a,b] must be continuous, and I cannot use rational/irrational tricks like in limit problems. does anyone have suggestions?

 

[foxofdesert]

should I just assume that x=c and x_n -> c as n->inf?