ReasoningAges 10-12

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 10-12

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 82° and 55°. Find the third angle.
82° · 55° · Missing triangle angles
2. Two interior angles of a triangle measure 33° and 40°. Find the third angle.
33° · 40° · Missing triangle angles
3. Two interior angles of a triangle measure 49° and 62°. Find the third angle.
49° · 62° · Missing triangle angles
4. Two interior angles of a triangle measure 58° and 59°. Find the third angle.
58° · 59° · Missing triangle angles
5. Two interior angles of a triangle measure 54° and 43°. Find the third angle.
54° · 43° · Missing triangle angles
6. Two interior angles of a triangle measure 34° and 64°. Find the third angle.
34° · 64° · Missing triangle angles
7. Two interior angles of a triangle measure 75° and 58°. Find the third angle.
75° · 58° · Missing triangle angles
8. Two interior angles of a triangle measure 38° and 41°. Find the third angle.
38° · 41° · Missing triangle angles
9. Two interior angles of a triangle measure 47° and 63°. Find the third angle.
47° · 63° · Missing triangle angles
10. Two interior angles of a triangle measure 38° and 31°. Find the third angle.
38° · 31° · Missing triangle angles
11. Two interior angles of a triangle measure 51° and 45°. Find the third angle.
51° · 45° · Missing triangle angles
12. Two interior angles of a triangle measure 34° and 60°. Find the third angle.
34° · 60° · Missing triangle angles

12 questions · Seed 70 · Age guidance, not curriculum certification

View the answer key

Worksheet StudioAnswer key · Ages 10-12

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 82° and 55°. Find the third angle.
43°
Angles in a triangle sum to 180°: 180° − (82° + 55°) = 43°.
2. Two interior angles of a triangle measure 33° and 40°. Find the third angle.
107°
Angles in a triangle sum to 180°: 180° − (33° + 40°) = 107°.
3. Two interior angles of a triangle measure 49° and 62°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (49° + 62°) = 69°.
4. Two interior angles of a triangle measure 58° and 59°. Find the third angle.
63°
Angles in a triangle sum to 180°: 180° − (58° + 59°) = 63°.
5. Two interior angles of a triangle measure 54° and 43°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (54° + 43°) = 83°.
6. Two interior angles of a triangle measure 34° and 64°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (34° + 64°) = 82°.
7. Two interior angles of a triangle measure 75° and 58°. Find the third angle.
47°
Angles in a triangle sum to 180°: 180° − (75° + 58°) = 47°.
8. Two interior angles of a triangle measure 38° and 41°. Find the third angle.
101°
Angles in a triangle sum to 180°: 180° − (38° + 41°) = 101°.
9. Two interior angles of a triangle measure 47° and 63°. Find the third angle.
70°
Angles in a triangle sum to 180°: 180° − (47° + 63°) = 70°.
10. Two interior angles of a triangle measure 38° and 31°. Find the third angle.
111°
Angles in a triangle sum to 180°: 180° − (38° + 31°) = 111°.
11. Two interior angles of a triangle measure 51° and 45°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (51° + 45°) = 84°.
12. Two interior angles of a triangle measure 34° and 60°. Find the third angle.
86°
Angles in a triangle sum to 180°: 180° − (34° + 60°) = 86°.

12 questions · Seed 70 · Age guidance, not curriculum certification

Worksheet StudioPractice pack · Ages 10-12

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 82° and 55°. Find the third angle.
82° · 55° · Missing triangle angles
2. Two interior angles of a triangle measure 33° and 40°. Find the third angle.
33° · 40° · Missing triangle angles
3. Two interior angles of a triangle measure 49° and 62°. Find the third angle.
49° · 62° · Missing triangle angles
4. Two interior angles of a triangle measure 58° and 59°. Find the third angle.
58° · 59° · Missing triangle angles
5. Two interior angles of a triangle measure 54° and 43°. Find the third angle.
54° · 43° · Missing triangle angles
6. Two interior angles of a triangle measure 34° and 64°. Find the third angle.
34° · 64° · Missing triangle angles
7. Two interior angles of a triangle measure 75° and 58°. Find the third angle.
75° · 58° · Missing triangle angles
8. Two interior angles of a triangle measure 38° and 41°. Find the third angle.
38° · 41° · Missing triangle angles
9. Two interior angles of a triangle measure 47° and 63°. Find the third angle.
47° · 63° · Missing triangle angles
10. Two interior angles of a triangle measure 38° and 31°. Find the third angle.
38° · 31° · Missing triangle angles
11. Two interior angles of a triangle measure 51° and 45°. Find the third angle.
51° · 45° · Missing triangle angles
12. Two interior angles of a triangle measure 34° and 60°. Find the third angle.
34° · 60° · Missing triangle angles

12 questions · Seed 70 · Age guidance, not curriculum certification

Worksheet StudioAnswer key · Ages 10-12

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 82° and 55°. Find the third angle.
43°
Angles in a triangle sum to 180°: 180° − (82° + 55°) = 43°.
2. Two interior angles of a triangle measure 33° and 40°. Find the third angle.
107°
Angles in a triangle sum to 180°: 180° − (33° + 40°) = 107°.
3. Two interior angles of a triangle measure 49° and 62°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (49° + 62°) = 69°.
4. Two interior angles of a triangle measure 58° and 59°. Find the third angle.
63°
Angles in a triangle sum to 180°: 180° − (58° + 59°) = 63°.
5. Two interior angles of a triangle measure 54° and 43°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (54° + 43°) = 83°.
6. Two interior angles of a triangle measure 34° and 64°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (34° + 64°) = 82°.
7. Two interior angles of a triangle measure 75° and 58°. Find the third angle.
47°
Angles in a triangle sum to 180°: 180° − (75° + 58°) = 47°.
8. Two interior angles of a triangle measure 38° and 41°. Find the third angle.
101°
Angles in a triangle sum to 180°: 180° − (38° + 41°) = 101°.
9. Two interior angles of a triangle measure 47° and 63°. Find the third angle.
70°
Angles in a triangle sum to 180°: 180° − (47° + 63°) = 70°.
10. Two interior angles of a triangle measure 38° and 31°. Find the third angle.
111°
Angles in a triangle sum to 180°: 180° − (38° + 31°) = 111°.
11. Two interior angles of a triangle measure 51° and 45°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (51° + 45°) = 84°.
12. Two interior angles of a triangle measure 34° and 60°. Find the third angle.
86°
Angles in a triangle sum to 180°: 180° − (34° + 60°) = 86°.

12 questions · Seed 70 · Age guidance, not curriculum certification