Indirect proof

Definition

An indirect proof is a method of proving the truth of a statement by assuming the opposite of what you want to prove and showing that this assumption leads to a contradiction. This technique is often used when direct proof is complicated or unclear, and it relies on the logical principle that if the negation of a statement results in an impossibility, then the original statement must be true.

5 Must Know Facts For Your Next Test

1. In an indirect proof, the goal is to demonstrate that the assumption of the opposite leads to a logical inconsistency or contradiction.
2. This method is particularly useful in cases where the direct approach may be cumbersome or difficult to navigate.
3. Indirect proofs often involve techniques such as proof by contradiction and using previously established statements or axioms.
4. The principle of excluded middle is fundamental to indirect proofs, stating that every statement must either be true or false.
5. Mastering indirect proof requires strong skills in logical reasoning and understanding how to manipulate assumptions effectively.

Review Questions

• How does an indirect proof establish the truth of a statement, and what logical principles support this method?
  • An indirect proof establishes the truth of a statement by assuming its negation and demonstrating that this leads to a contradiction. This approach relies on logical principles such as proof by contradiction and the principle of excluded middle, which asserts that any statement must either be true or false. By showing that the assumption cannot hold without leading to an impossibility, it concludes that the original statement must indeed be true.
• Compare and contrast indirect proof with direct proof. What are the advantages and limitations of each method?
  • Indirect proof differs from direct proof in its approach; while direct proof establishes truth through straightforward arguments, indirect proof starts with an assumption of falsehood. The advantage of indirect proof lies in its ability to simplify complex problems where direct evidence may be hard to find. However, direct proofs can sometimes be clearer and more intuitive. The choice between these methods often depends on the specific context of the problem being addressed.
• Evaluate the effectiveness of using indirect proof in mathematical arguments. What impact does this method have on problem-solving strategies?
  • Indirect proof is highly effective in mathematical arguments, especially when faced with complex statements where direct methods are impractical. By allowing mathematicians to explore assumptions that lead to contradictions, it opens up new avenues for reasoning and can uncover deeper truths about relationships within propositions. This method not only enhances problem-solving strategies but also enriches understanding by highlighting the interconnectedness of logical statements in mathematics.

Related terms

Proof by Contradiction: A specific type of indirect proof where one assumes that a statement is false and demonstrates that this assumption leads to a contradiction.

Logical Negation: The process of flipping the truth value of a statement, where the negation of a true statement is false and vice versa.

Direct Proof:

A method of proving a statement by straightforwardly showing that it is true through logical deductions and previously established facts.