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Grade 8 Triangle Angle Sum & Exterior Angles Worksheet with Answers

Grade 8 (US) · Year 9 (UK) · Class 8 (India) · Math

Unit 4: GeometryAngles & TrianglesCCSS 8.G.A.5

Using the 180° angle sum and the exterior angle theorem to find missing angles. This free middle school worksheet has 12 questions and a full answer key. Print it or save it as a PDF — every answer is checked by code, not guessed.

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Triangle Angle Sum & Exterior Angles

Grade 8 · Math

Score___ / 12

Name

Class

Date

Learning goal: Angles & Triangles · CCSS 8.G.A.5

Directions: Use the pictures to help you. Solve each problem. Show your work, then write your answer in the box.

Example

Two angles of a triangle are 41° and 109°. Find the third angle.

Answer: 30°

RememberAngles in a triangle

abca + b + c = 180°The three angles of any trianglealways add up to 180°.Exterior angle = sum of the two far angles
  1. 1

    An isosceles triangle has a vertex angle of 120°. What is each base angle?

    Show your work
    Answer
  2. 2

    A triangle has interior angles 93° and 32°. What is the exterior angle at the third vertex?

    93°32°?
    Answer
  3. 3

    An isosceles triangle has a vertex angle of 132°. What is each base angle?

    Show your work
    Answer
  4. 4

    Two angles of a triangle are 50° and 29°. Find the third angle.

    50°29°?
    Answer
  5. 5

    Two angles of a triangle are 55° and 79°. Find the third angle.

    55°79°?
    Answer
  6. 6

    A triangle has interior angles 28° and 104°. What is the exterior angle at the third vertex?

    28°104°?
    Answer
  7. 7

    Two angles of a triangle are 67° and 64°. Find the third angle.

    67°64°?
    Answer
  8. 8

    Two angles of a triangle are 66° and 30°. Find the third angle.

    66°30°?
    Answer
  9. 9

    Two angles of a triangle are 36° and 60°. Find the third angle.

    36°60°?
    Answer
  10. 10

    An isosceles triangle has a vertex angle of 28°. What is each base angle?

    Show your work
    Answer
  11. 11

    A triangle has interior angles 27° and 25°. What is the exterior angle at the third vertex?

    27°25°?
    Answer
  12. 12

    An isosceles triangle has a vertex angle of 66°. What is each base angle?

    Show your work
    Answer

★ Challenge corner for fast finishers

Each scale balances. Scale 1: circle + square = 25; scale 2: square + triangle = 18; scale 3: circle + circle = 24. What is one triangle worth?

25Scale 118Scale 224Scale 3

How did I do?

Teacher's comments

Free printable worksheets with answer keys · aethelsystems.com/worksheets/math/8/triangle_angles

Answer key (click to show)
  1. 1.30°
  2. 2.125°
  3. 3.24°
  4. 4.101°
  5. 5.46°
  6. 6.132°
  7. 7.49°
  8. 8.84°
  9. 9.84°
  10. 10.76°
  11. 11.52°
  12. 12.57°
  13. ★5 (challenge corner)

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Guides for teaching Triangle Angle Sum & Exterior Angles

What Are the Common Core and NGSS Standards? A Parent's GuideWhat learning standards are, how to read a code like 3.NF.A.1 or MS-PS1-2, what each grade covers in Math, English and Science, and how to use them at home.

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