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Geometry: Proofs

Grade 9 · Math · Free lesson

Hey there! Today, we are going to learn how to think like mathematical detectives by exploring the world of geometric proofs.

In geometry, a proof is a step-by-step argument that shows why a statement must always be true. Instead of just guessing, we use established facts, definitions, and rules to build our case.

pi

Every proof starts with 'Given' facts (our clues) and a 'Prove' statement (our goal). We connect them using a chain of logical steps, where each step must have a solid reason to back it up.

1. Given Clue2. Reason3. Proven!

Think of a proof like a bridge. If we know that Segment A equals Segment B, and Segment B equals Segment C, then Segment A must equal Segment C. This is called the Transitive Property!

ABCA = C
✏️ Worked example

Given that Angle 1 and Angle 2 are vertical angles, prove that Angle 1 is congruent to Angle 2.

  1. Identify the given information: Angle 1 and Angle 2 are vertical angles formed by two intersecting lines.
  2. Recall the definition of vertical angles: they are opposite each other where two straight lines cross.
  3. State the Vertical Angles Theorem: Vertical angles are always equal to each other.
  4. Write down the final statement: Therefore, Angle 1 is congruent to Angle 2.
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