6. i et f : a on a set e e d ? be function defined d in rt with d. (a) define what it means for f to be continnous at c, and write down a couple of other 3 equivalent conditions (b) let f() -0if is rational, and /(n)- rif r is irrational. show that fis discontimous how that f is discontinuous at every nonzero number but that f is continuous at 0. give all details.