M201: First graded homework
For each question, state the proposition and write a formal proof.

(3 points.)
If x is an integer,
then x^{2}  3x +6 is even.

(3 points.)
If x is an integer,
and x^{2}  3 is odd,
then x is even.

(3 points.)
There are no integers x, y
such that 20x  15y = 1.
