(1.8) (2 points) Let f be a real function on [0,2], then f is Riemann integrable on [0,2] if and only if|f| is Riemann integrable on [0, 2].

(4) (20 points) Every rational t can be written in the form, where n> 0, and m and n are integers without any common divisors. When a = O, we take n = 1. Consider the function f defined on R by Prove that f is continuous at every irrational point, and that f has a discontinuity at every rational point.