Direct Proof

Learn to construct direct proofs by logically deriving conclusions from given hypotheses using definitions, axioms, and previously proven theorems.

Advanced25 minLesson
Printable worksheet

Definition

A direct proof is a method of proving a mathematical statement by starting with known facts (the hypothesis) and using logical steps to arrive at the conclusion.
The structure of a direct proof:
  1. 1.Assume the hypothesis is true
  2. 2.Apply definitions, axioms, and theorems
  3. 3.Derive the conclusion through logical reasoning
If we want to prove "If PP, then QQ" (written P⇒QP \Rightarrow Q):
  • Start by assuming PP is true
  • Use logical deductions to show QQ must also be true
P⇒Q means: whenever P is true, Q is also trueP \Rightarrow Q \text{ means: whenever } P \text{ is true, } Q \text{ is also true}

Try it now

What is the first step in a direct proof?

Worked Examples

Prove: If aa and bb are even integers, then a+ba + b is even.
1

State the hypothesis

Assume aa and bb are even integers → Given: aa and bb are even
2

Apply the definition of even

An even integer can be written as 2k2k for some integer kk → a=2ma = 2m and b=2nb = 2n for integers m,nm, n
3

Add the two numbers

a+b=2m+2na + b = 2m + 2n
4

Factor out the common factor

a+b=2(m+n)a + b = 2(m + n)
5

Apply the definition of even again

Since m+nm + n is an integer, 2(m+n)2(m + n) is even by definition → a+ba + b is even

Common Mistakes

Using specific examples instead of general proof

Why it's wrong: Showing that 2+4=62 + 4 = 6 (even) proves nothing about ALL even numbers. A proof must work for every possible case.

Correct: Use variables and definitions. Write 'Let a=2ma = 2m for some integer mm' to represent ANY even number.

Assuming what you want to prove

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

Correct: Start only from the hypothesis and work forward. Never write 'Since a+ba + b is even...' until you have proven it.

Skipping logical steps

Why it's wrong: Each step must follow logically from previous steps. Gaps in reasoning can hide errors.

Correct: Justify every step with a definition, axiom, or previously proven theorem.

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

17 problems
Problem 1 of 17
Easy
What is the first step in a direct proof?

Why It Matters

Direct proof is the most fundamental proof technique in mathematics:
  • Foundation of mathematics: Every theorem you use was proven using direct proof or related methods
  • Computer science: Algorithm correctness proofs often use direct reasoning
  • Legal arguments: Lawyers construct arguments from evidence to conclusion
  • Scientific method: Deriving predictions from hypotheses uses the same logical structure
  • Problem solving: Breaking complex problems into logical steps
Mastering direct proof develops rigorous thinking that applies far beyond mathematics!

Real World Applications

Software Verification

Programmers use direct proof logic to verify that their code works correctly for all inputs.

Example:

To prove a sorting algorithm always produces sorted output, we show: if the input is a list, the output has the same elements in non-decreasing order.

1Try It Yourself

A program doubles every number in a list. We need to prove: if all input numbers are positive, all output numbers are positive.

Complete this direct proof.

Step 1: Choose the right answer

If x>0x > 0, then 2x>?2x > ?

Legal Reasoning

Lawyers construct arguments that logically lead from evidence to conclusions.

Example:

If the contract states payment is due upon delivery, and delivery occurred on March 1, then payment was due on March 1.

2Try It Yourself

Law: All citizens over 18 can vote. Fact: Alex is a citizen. Fact: Alex is 21 years old.

Prove that Alex can vote.

Step 1: Choose the right answer

Which comparison of Alex's age with 18 completes the argument?

Key Takeaways

  • 1A direct proof assumes the hypothesis is true and derives the conclusion through logical steps
  • 2Use definitions to translate words into mathematical expressions (e.g., even = 2k2k)
  • 3Each step must follow logically from previous steps using algebra, definitions, or theorems
  • 4The proof ends when you reach the exact form of the conclusion
  • 5QED (quod erat demonstrandum) marks the end of a proof

Frequently Asked Questions

Try direct proof first for positive statements like 'If A, then B.' If you get stuck, consider proof by contradiction or contrapositive. Direct proof works well when you can manipulate the hypothesis algebraically.
Try direct proof first for positive statements like 'If A, then B.' If you get stuck, consider proof by contradiction or contrapositive. Direct proof works well when you can manipulate the hypothesis algebraically.
QED stands for 'quod erat demonstrandum' (Latin for 'which was to be demonstrated'). It signals the proof is complete. Some mathematicians use a square symbol (tombstone) instead.
Each step should be justified by a definition, property, or theorem. If you skip steps, you risk hiding errors. When in doubt, include more detail.

Glossary

Direct proof
A proof method that assumes the hypothesis and uses logical deduction to derive the conclusion
Hypothesis
The 'if' part of a conditional statement; what we assume to be true
Conclusion
The 'then' part of a conditional statement; what we want to prove
QED
Abbreviation for 'quod erat demonstrandum,' marking the end of a proof
Theorem
A mathematical statement that has been proven to be true
Axiom
A statement accepted as true without proof; a starting point for reasoning
Today's challengeOne problem, three tries

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