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Learn/logic/Implications & The If and Only If

Lesson subsection

Implications & The If and Only If

Read the explanation, try the on-paper prompts, then explain the idea in your own words. Use AI feedback as a mentor, not a shortcut.

10–20 min focusProof-first mindset

Best flow: read → think on paper → write a short explanation → refine with feedback.

Reading

Core explanation

Students often misunderstand the implication P → Q. It does not mean P "causes" Q.
It simply means: if P is true, then Q must also be true.

The only time P → Q is false is when P is true and Q is false.

An if and only if statement (P ↔ Q) means:

  • P implies Q
  • Q implies P
    Both must hold.

IFF statements are very common in definitions.

TL;DR — key idea

Implications are only false when the hypothesis is true and the conclusion is false. IFF means two statements imply each other.

Try these in your notebook

Don’t skip this – writing proofs or explanations on paper is where most of the learning actually happens.

  • 1

    Give an example of a true implication and explain why it is true. Then give an example of a statement that is true "if and only if" and explain both directions.

Once you’ve sketched some ideas, summarize the main insight in the reflection box on the right.

Check your understanding

In 3–6 sentences, explain the core idea of this subsection as if you were teaching a friend who hasn’t seen it. Focus on the logic, not just the final statements.

AI is optional. Use it to spot gaps and sharpen your wording, not to replace your own thinking.