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IB DP Maths AA SL Study Notes

1.5.1 Direct and Indirect Proofs

Direct Proof

Direct proof stands as a fundamental method in mathematics, affirming the truth of a statement through logical deductions and established axioms or theorems.

Definition and Basics

  • Direct Proof: This method validates a statement by establishing a logical sequence of deductions from given assumptions, using definitions, axioms, and previously proven theorems.
  • Logical Sequence: The proof must seamlessly transition from one logical step to the next, ensuring a coherent progression from the hypothesis to the conclusion.

Steps in Constructing a Direct Proof

  • Statement Identification: Clearly pinpoint the statement that requires proving.
  • Assumptions: Begin with the assumptions or given information that the proof will build upon.
  • Logical Progression: Employ logical steps and previously proven theorems to affirm the statement’s truth. This method is closely related to the approach used in proving sequences and series, where a similar foundational understanding is necessary.

Example: Direct Proof

Statement: If n is an even integer, then n squared is also even.


  • Assumption: Let n be an even integer.
  • Definition: An even integer can be expressed as n = 2k, where k is an integer.
  • Calculation: n squared = (2k) squared = 4k squared = 2(2k squared).
  • Conclusion: Since n squared is divisible by 2, it is also an even integer.

For those interested in expanding their understanding of mathematical proofs, exploring the basics of mathematical induction can provide further insights into direct proofs and their applications.

Indirect Proof

Indirect proof, in contrast to direct proof, substantiates a statement by demonstrating that its negation is false. This category encompasses proof by contradiction and contrapositive proof.

1. Proof by Contradiction

Proof by contradiction, or "reductio ad absurdum", validates a statement by assuming its negation is true and demonstrating that this assumption leads to a contradiction.

Steps in Proof by Contradiction

  • Assume the Opposite: Begin by assuming that the negation of the statement is true.
  • Find a Contradiction: Employ logical reasoning to derive a contradiction from this assumption.
  • Conclusion: Conclude that the original statement must be true since its negation leads to a contradiction.

Example: Proof by Contradiction

Statement: There are infinitely many prime numbers.


  • Assume the Opposite: Suppose there are finitely many prime numbers.
  • Contradiction: Multiplying all the prime numbers together and adding 1 yields a number not divisible by any of the primes, contradicting the assumption.
  • Conclusion: Therefore, there must be infinitely many prime numbers. This method is akin to proving the polar form and de Moivre's theorem, where a contradiction establishes the foundation of the proof.
IB Maths Tutor Tip: Understanding the essence of direct and indirect proofs sharpens logical thinking, crucial for tackling complex mathematical arguments and fostering a deeper grasp of mathematical concepts.

2. Contrapositive Proof

Contrapositive proof validates a statement "If P, then Q" by affirming "If not Q, then not P".

Steps in Contrapositive Proof

1. Identify the Contrapositive: Determine the contrapositive of the statement.

2. Prove the Contrapositive: Utilize logical reasoning to validate the contrapositive.

3. Conclusion: Affirm that the original statement is true if the contrapositive is true.

Example: Contrapositive Proof

Statement: If n squared is odd, then n is odd.


  • Contrapositive: If n is not odd (i.e., even), then n squared is not odd (i.e., even).
  • Proof of Contrapositive: If n = 2k (even), then n squared = (2k) squared = 4k squared = 2(2k squared), which is even.
  • Conclusion: The original statement is true. Similar logical structures are applied in proving inequalities, highlighting the versatility of indirect proofs.

Applications in Various Mathematical Domains


  • Direct Proof: Proving algebraic identities and theorems directly using algebraic manipulations.
  • Proof by Contradiction: Demonstrating that assuming the non-existence of a solution leads to a contradiction.
  • Contrapositive Proof: Proving algebraic implications by establishing the truth of their contrapositives.

The principles of direct and indirect proofs are also fundamental in understanding permutations, where the arrangement of elements in a particular order can be explored through these logical frameworks.


  • Direct Proof: Establishing geometric properties using definitions and previously proven properties.
  • Proof by Contradiction: Proving the uniqueness of a geometric entity by assuming the existence of a second entity and deriving a contradiction.
  • Contrapositive Proof: Proving geometric implications by validating their contrapositives.
IB Tutor Advice: Practice identifying the type of proof needed for different problems, as it's key to crafting effective solutions in exams. Familiarise yourself with examples to build confidence and skill.

Number Theory

  • Direct Proof: Verifying properties of numbers using arithmetic and previously established results.
  • Proof by Contradiction: Proving the infinitude or non-existence of certain types of numbers.
  • Contrapositive Proof: Demonstrating numerical properties by affirming the truth of their contrapositives.


Yes, there are statements that might be challenging or impossible to prove using direct proof due to the lack of a straightforward path from the assumptions to the conclusion. In such cases, mathematicians might resort to indirect proof methods, such as proof by contradiction or contrapositive proof, which might offer alternative pathways to establish the truth of the statement. Some statements, particularly in higher mathematics, might be inherently non-constructive, making direct proof unsuitable and necessitating the use of indirect proof methods.

A proof by contradiction is valid if the assumption of the negation of the statement logically leads to a contradiction, and every step in the proof adheres to logical and mathematical principles. It’s crucial to ensure that the contradiction arises necessarily from the assumption and not due to any logical fallacies or incorrect use of mathematical properties. Rigorous checking of each step, peer review, and verifying the logical coherence of the argument ensure the validity of a proof by contradiction. If the contradiction is established correctly, the original statement is affirmed as true according to the principles of classical logic.

Choosing a method of proof often depends on the nature of the statement and the available information. Direct proof is often the first choice when the statement can be verified in a straightforward manner using known facts and theorems. Proof by contradiction might be suitable when assuming the opposite of the statement leads to logical inconsistencies. Contrapositive proof is apt when proving the contrapositive is simpler than proving the original statement. The choice might also be influenced by the clarity and elegance of the argument, ensuring that the proof is not only valid but also easily comprehensible to others in the mathematical community.

No, a statement cannot be proven true and false using different methods of proof if the proofs are correct. Mathematical statements are either true or false, not both. If different methods seem to provide contradictory results, it indicates that at least one of the proofs is incorrect. It’s essential to meticulously check each step of the proofs for validity and consistency with mathematical principles. A correct proof, regardless of the method used, should always yield the same truth value for a particular statement, adhering to the principle of non-contradiction in classical logic.

Understanding and employing various proof methods in mathematics is crucial because different theorems or statements might require distinct approaches for verification. Direct proof might be suitable for certain propositions, while indirect proof might be more apt for others, depending on the available information and the nature of the statement. Moreover, different proof methods can provide diverse insights into the problem at hand, enhancing the comprehensiveness and depth of understanding. It also equips students and mathematicians with the flexibility to choose the most efficient and clear proof method for a particular statement, facilitating better communication of mathematical ideas.

Practice Questions

Prove that the square of any odd integer is also odd.

To prove that the square of any odd integer is odd, let's assume that n is an odd integer. By definition, an odd integer can be expressed in the form n = 2k + 1, where k is an integer. Squaring n, we get n squared = (2k + 1) squared = 4k squared + 4k + 1 = 2(2k squared + 2k) + 1. The expression 2k squared + 2k is an integer, let's call it m. So, n squared = 2m + 1, which is the standard form for an odd number. Therefore, the square of any odd integer is also odd.

Using proof by contradiction, demonstrate that there is no smallest positive rational number.

To prove that there is no smallest positive rational number using proof by contradiction, let's assume the opposite: that there is a smallest positive rational number, and let's call it r. Since r is rational, it can be expressed as a fraction r = a/b, where a and b are coprime integers. Now, consider the rational number s = a/(2b). Clearly, s is positive and rational, and s = (1/2) * (a/b) = (1/2) * r. Since s is half of r, it is smaller than r, which contradicts our initial assumption that r is the smallest positive rational number. Therefore, there is no smallest positive rational number.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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