Introduction to Mathematical Proofs

Learn the fundamentals of mathematical proofs and how to construct logical arguments.

Advanced25 minLesson
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Definition

A mathematical proof is a logical argument that demonstrates why a mathematical statement must be true. Proofs use established facts, definitions, and logical rules to reach an undeniable conclusion.
A proof consists of:
  • Given information (premises or hypotheses)
  • Logical steps (each justified by a rule or previously proven fact)
  • Conclusion (the statement being proven)
The structure of a simple proof:
If P then Q\text{If } P \text{ then } Q
Where PP is our starting assumption and QQ is what we want to prove.

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What is a mathematical proof?

Worked Examples

Prove that the sum of any two even numbers is even.
1

Define what even means

A number nn is even if n=2kn = 2k for some integer kk → Definition established
2

Let the two even numbers be aa and bb

By definition: a=2ma = 2m and b=2nb = 2n where m,nm, n are integers → Express in terms of definition
3

Add the two numbers

a+b=2m+2n=2(m+n)a + b = 2m + 2n = 2(m + n) → Factor out 2
4

Identify the form

2(m+n)2(m + n) is 2×(integer)2 \times (\text{integer}), which is the definition of even → Matches even definition
5

State conclusion

Therefore, a+ba + b is even → QED

Common Mistakes

Using specific examples instead of general proof

Why it's wrong: Showing that 2+4=62 + 4 = 6 is even only proves that ONE case works, not ALL cases.

Correct: Use variables (2m+2n2m + 2n) to represent ANY two even numbers.

Assuming what you want to prove

Why it's wrong: You cannot use the conclusion in your reasoning - that is circular logic.

Correct: Start only from given information and definitions, then derive the conclusion.

Missing justification for steps

Why it's wrong: Every step must be justified by a definition, axiom, or previously proven theorem.

Correct: Always state WHY each step follows from the previous one.

Confusing implication direction

Why it's wrong: If P⇒QP \Rightarrow Q, knowing QQ does NOT tell us PP is true.

Correct: Be careful: converse and inverse of a true statement may be false.

Interactive Visual

Truth Table

P ∧ Q
PQP ∧ Q
FFF
FTF
TFF
TTT

T= TrueF= False

Interactive Sandbox

Expression Calculator

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Practice Problems

16 problems
Problem 1 of 16
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What is a mathematical proof?

Why It Matters

Mathematical proofs are the foundation of all mathematics and critical thinking:
  • Certainty: Unlike science which relies on evidence, proofs provide absolute certainty
  • Computer Science: Program verification and algorithm correctness rely on proofs
  • Law: Legal arguments follow similar logical structures
  • Philosophy: Logical reasoning is essential for philosophical arguments
  • Problem Solving: Learning to prove develops systematic thinking skills
Every theorem you use in mathematics was proven by someone using these techniques!

Real World Applications

Computer Science: Algorithm Correctness

Software engineers use proofs to verify that programs work correctly for all inputs.

Example:

Proving that a sorting algorithm always produces a sorted list usually uses mathematical induction.

1Try It Yourself

A loop repeats nn times. Each iteration doubles a counter starting at 1.

Prove the final value is 2n2^n.

Step 1: Write the mathematical expression

After kk iterations, the value is:

Cryptography: Security Proofs

Cryptographers use proofs to show that breaking a code is at least as hard as solving a well-studied math problem.

Example:

RSA encryption is safe as long as factoring very large numbers stays hard. Nobody knows a fast method, but nobody has proven that none exists either.

2Try It Yourself

If you can factor the product of two primes, you can break the code.

Why is 15 easier to factor than 437?

Step 1: Choose the right answer

To find a factor, you only test primes up to the square root. Which comparison is true?

Legal Reasoning: Building Arguments

Lawyers construct logical arguments similar to mathematical proofs.

Example:

Legal reasoning follows 'If the evidence shows X, and X implies guilt, then the defendant is guilty.'

3Try It Yourself

Rule: All contracts require consideration (exchange of value). Person A signed a document but received nothing.

Is this a valid contract?

Step 1: Choose the right answer

Apply the logical rule:

Key Takeaways

  • 1A mathematical proof is a logical argument showing why a statement must be true
  • 2Direct proof: assume the hypothesis, derive the conclusion step by step
  • 3Proof by contradiction: assume the opposite is true, reach a contradiction
  • 4Every step must be justified by a definition, axiom, or proven theorem
  • 5Proofs must work for ALL cases, not just specific examples

Frequently Asked Questions

An axiom is a statement accepted as true without proof. A theorem is a statement that has been proven using axioms and other theorems.
An axiom is a statement accepted as true without proof. A theorem is a statement that has been proven using axioms and other theorems.
Testing examples only shows specific cases work. A proof guarantees ALL cases work. For example, we cannot test infinitely many numbers, but a proof covers them all.
QED stands for 'quod erat demonstrandum' (Latin for 'what was to be demonstrated'). It marks the end of a proof.

Glossary

Proof
A logical argument demonstrating that a mathematical statement is true
Theorem
A mathematical statement that has been proven to be true
Axiom
A statement accepted as true without proof; a starting point for reasoning
Hypothesis
The starting assumption in a conditional statement (the 'if' part)
Conclusion
The result that follows from the hypothesis (the 'then' part)
Contradiction
A logical impossibility; two statements that cannot both be true
QED
Abbreviation marking the end of a proof (quod erat demonstrandum)
Today's challengeOne problem, three tries

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