Useful resource. It doesn't stress enough the value of direct proofs for intuition enough to my liking. Example: in section 6.4, it is easy to prove that 6a^2

Useful resource. It doesn't stress enough the value of direct proofs for intuition enough to my liking. Example: in section 6.4, it is easy to prove that 6a^2-2a+7 is even iff a is odd, and the direct proof gives a better intuition that the proof by contradiction and contrapositive proofs described.