This lesson is about proving something by showing that it's opposite cannot happen. If the opposite can't happen, then the opposite of the opposite must happen. Glad that's cleared up.

Proof by Contradiction

Khan Academy. Indirect proof that the square root of two is irrational. Duration_7_46

Geometry 1(A) apply mathematics to problems arising in everyday life, society, and the workplace;

Geometry 1(B) use a problem-solving model that incorporates analyzing given information, formulating a plan or strategy, determining a solution, justifying the solution, and evaluating the problem-solving process and the reasonableness of the solution;

Geometry 1(D) communicate mathematical ideas, reasoning, and their implications using multiple representations, including symbols, diagrams, graphs, and language as appropriate;

Geometry 1 (G) display, explain, or justify mathematical ideas and arguments using precise mathematical language in written or oral communication