4 Proof by? (a) Prove that if for any two integers x and y, if 10 does not divide xy, then 10 does not divide x and 10 does not divide y. In notation: (Vx,y E Z) (10 fxy) => ((10fx) /\\ (lOfy)). What proof technique did you use? (b) Prove or disprove the contrapositive. (c) Prove or disprove the converse