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Geometry · Chapter 5 · Lesson 5.10 · Printable PDF

Indirect Proof Worksheet with Answer Key

6 problems on indirect proof in geometry: assuming the opposite and reaching a contradiction, with triangle inequalities, fences, angles and parallel lines. 2 pages, with a worked answer key.

6 problems2 pagesanswer key
Printable Indirect proof worksheet with answer key - Geometry (Page 1 PDF preview)
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Try one first: a free Chapter 1 worksheet (PDF, no sign-in).

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What's on the worksheet

  1. Use an indirect argument to decide whether certain triangles can exist.
  2. Explain why the longest side is shorter than 6 and list every triangle with perimeter 12.
  3. Show Ava's and Emma's measurements for a point inside a triangle must be wrong.
  4. Prove indirectly that Noah's four-sided plot with given sides cannot exist.
  5. Write an indirect proof that one acute angle of a right triangle exceeds 45 degrees.
  6. Write an indirect two-column proof that a line meets a second parallel line.

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The free lesson teaches it with worked examples and practice problems.

Indirect proof: the lesson ›

More Chapter 5 worksheets: Triangle segments, centers and inequalities