ReasoningAges 13-15

Missing triangle angles

Use the 180° interior angle sum of a triangle to deduce an unknown angle. Review suitability for your learners before teaching.

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Age band:Ages 10-12 Ages 13-15

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Worksheet StudioPractice pack · Ages 13-15

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 82° and 27°. Find the third angle.
82° · 27° · Missing triangle angles
2. Two interior angles of a triangle measure 81° and 72°. Find the third angle.
81° · 72° · Missing triangle angles
3. Two interior angles of a triangle measure 39° and 39°. Find the third angle.
39° · 39° · Missing triangle angles
4. Two interior angles of a triangle measure 34° and 47°. Find the third angle.
34° · 47° · Missing triangle angles
5. Two interior angles of a triangle measure 64° and 41°. Find the third angle.
64° · 41° · Missing triangle angles
6. Two interior angles of a triangle measure 70° and 60°. Find the third angle.
70° · 60° · Missing triangle angles
7. Two interior angles of a triangle measure 75° and 40°. Find the third angle.
75° · 40° · Missing triangle angles
8. Two interior angles of a triangle measure 78° and 75°. Find the third angle.
78° · 75° · Missing triangle angles
9. Two interior angles of a triangle measure 33° and 57°. Find the third angle.
33° · 57° · Missing triangle angles
10. Two interior angles of a triangle measure 53° and 60°. Find the third angle.
53° · 60° · Missing triangle angles
11. Two interior angles of a triangle measure 58° and 28°. Find the third angle.
58° · 28° · Missing triangle angles
12. Two interior angles of a triangle measure 74° and 49°. Find the third angle.
74° · 49° · Missing triangle angles

12 questions · Seed 58 · Age guidance, not curriculum certification

View the answer key

Worksheet StudioAnswer key · Ages 13-15

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 82° and 27°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (82° + 27°) = 71°.
2. Two interior angles of a triangle measure 81° and 72°. Find the third angle.
27°
Angles in a triangle sum to 180°: 180° − (81° + 72°) = 27°.
3. Two interior angles of a triangle measure 39° and 39°. Find the third angle.
102°
Angles in a triangle sum to 180°: 180° − (39° + 39°) = 102°.
4. Two interior angles of a triangle measure 34° and 47°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (34° + 47°) = 99°.
5. Two interior angles of a triangle measure 64° and 41°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (64° + 41°) = 75°.
6. Two interior angles of a triangle measure 70° and 60°. Find the third angle.
50°
Angles in a triangle sum to 180°: 180° − (70° + 60°) = 50°.
7. Two interior angles of a triangle measure 75° and 40°. Find the third angle.
65°
Angles in a triangle sum to 180°: 180° − (75° + 40°) = 65°.
8. Two interior angles of a triangle measure 78° and 75°. Find the third angle.
27°
Angles in a triangle sum to 180°: 180° − (78° + 75°) = 27°.
9. Two interior angles of a triangle measure 33° and 57°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (33° + 57°) = 90°.
10. Two interior angles of a triangle measure 53° and 60°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (53° + 60°) = 67°.
11. Two interior angles of a triangle measure 58° and 28°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (58° + 28°) = 94°.
12. Two interior angles of a triangle measure 74° and 49°. Find the third angle.
57°
Angles in a triangle sum to 180°: 180° − (74° + 49°) = 57°.

12 questions · Seed 58 · Age guidance, not curriculum certification

Worksheet StudioPractice pack · Ages 13-15

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 82° and 27°. Find the third angle.
82° · 27° · Missing triangle angles
2. Two interior angles of a triangle measure 81° and 72°. Find the third angle.
81° · 72° · Missing triangle angles
3. Two interior angles of a triangle measure 39° and 39°. Find the third angle.
39° · 39° · Missing triangle angles
4. Two interior angles of a triangle measure 34° and 47°. Find the third angle.
34° · 47° · Missing triangle angles
5. Two interior angles of a triangle measure 64° and 41°. Find the third angle.
64° · 41° · Missing triangle angles
6. Two interior angles of a triangle measure 70° and 60°. Find the third angle.
70° · 60° · Missing triangle angles
7. Two interior angles of a triangle measure 75° and 40°. Find the third angle.
75° · 40° · Missing triangle angles
8. Two interior angles of a triangle measure 78° and 75°. Find the third angle.
78° · 75° · Missing triangle angles
9. Two interior angles of a triangle measure 33° and 57°. Find the third angle.
33° · 57° · Missing triangle angles
10. Two interior angles of a triangle measure 53° and 60°. Find the third angle.
53° · 60° · Missing triangle angles
11. Two interior angles of a triangle measure 58° and 28°. Find the third angle.
58° · 28° · Missing triangle angles
12. Two interior angles of a triangle measure 74° and 49°. Find the third angle.
74° · 49° · Missing triangle angles

12 questions · Seed 58 · Age guidance, not curriculum certification

Worksheet StudioAnswer key · Ages 13-15

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 82° and 27°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (82° + 27°) = 71°.
2. Two interior angles of a triangle measure 81° and 72°. Find the third angle.
27°
Angles in a triangle sum to 180°: 180° − (81° + 72°) = 27°.
3. Two interior angles of a triangle measure 39° and 39°. Find the third angle.
102°
Angles in a triangle sum to 180°: 180° − (39° + 39°) = 102°.
4. Two interior angles of a triangle measure 34° and 47°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (34° + 47°) = 99°.
5. Two interior angles of a triangle measure 64° and 41°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (64° + 41°) = 75°.
6. Two interior angles of a triangle measure 70° and 60°. Find the third angle.
50°
Angles in a triangle sum to 180°: 180° − (70° + 60°) = 50°.
7. Two interior angles of a triangle measure 75° and 40°. Find the third angle.
65°
Angles in a triangle sum to 180°: 180° − (75° + 40°) = 65°.
8. Two interior angles of a triangle measure 78° and 75°. Find the third angle.
27°
Angles in a triangle sum to 180°: 180° − (78° + 75°) = 27°.
9. Two interior angles of a triangle measure 33° and 57°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (33° + 57°) = 90°.
10. Two interior angles of a triangle measure 53° and 60°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (53° + 60°) = 67°.
11. Two interior angles of a triangle measure 58° and 28°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (58° + 28°) = 94°.
12. Two interior angles of a triangle measure 74° and 49°. Find the third angle.
57°
Angles in a triangle sum to 180°: 180° − (74° + 49°) = 57°.

12 questions · Seed 58 · Age guidance, not curriculum certification