Let n be an integer. Prove that 3n-11 is odd if and only if Let n be an integer. Prove that n is even if and only if n is even. Let n be an integer. Prove that n is odd if and only if n is odd. Let n be an integer. Prove that 2n-n-1=0 if and only if 3n2-n-2=0. Give a proof of n is even. Let ne Z. Then n-3 is even if and only if n +4 is odd. using (a) two direct proofs. (b) one direct proof and one proof by contrapositive. (c) two proofs by contrapositive. Let z be a real number. Prove that if +5z+10, then