Read Sections 2.4, 2.6,

p.523 #6.   p.15 #27.   p.24 #7.   p.31 #13, 14.  p.37 #8, 9.  p.#45. 8.  p.53 #3, 5, 9, 12.

Show that the common factors of a and b are precisely the factors of (a,b).