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Geometry · Chapter 2 · Lesson 2.10 · Printable PDF

Proving the Triangle Angle Sum Worksheet with Answer Key

4 problems on proving the triangle angle sum: the Exterior Angle Theorem, angle bisectors outside a triangle, and two-column proofs using parallel lines. 3 pages, with a worked answer key.

4 problems3 pagesanswer key
Printable Proving the triangle angle sum 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. Decide whether each citation of the Exterior Angle Theorem is correct and say what it gives.
  2. Find angles where bisectors of two exterior angles meet at P.
  3. Write a two-column proof that an angle bisector makes an angle congruent to angle B.
  4. Prove the three angles add to 180 degrees using parallel lines DF and DE.

Teach it first

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

Proving the triangle angle sum: the lesson ›

More Chapter 2 worksheets: Angle pairs, parallel lines and first proofs