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Exercise 497Logic, sets and proofs › Proof techniquesL1Added 3 Nov 2026

Contrapositive with primes

#contrapositive#prime#divisibility

Statement

Prove: if pp is prime and p>2p>2, then pp is odd.

Hint

The contrapositive: if pp is even, then pp is not prime or p≤2p\le2.

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Solution

Contrapositive. "If pp is prime and p>2p>2, then pp is odd" is equivalent to "if pp is even (and prime), then p≤2p\le2".

Proof. If pp is even prime, p=2kp=2k for some kk. Since pp is prime and 2∣p2\mid p, either p=2p=2 (so k=1k=1) or pp is composite. Primality forces p=2p=2.

Conclusion. All primes p>2p>2 are odd.