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 52° and 43°. Find the third angle.
52° · 43° · Missing triangle angles
2. Two interior angles of a triangle measure 29° and 38°. Find the third angle.
29° · 38° · Missing triangle angles
3. Two interior angles of a triangle measure 76° and 63°. Find the third angle.
76° · 63° · Missing triangle angles
4. Two interior angles of a triangle measure 29° and 45°. Find the third angle.
29° · 45° · Missing triangle angles
5. Two interior angles of a triangle measure 69° and 36°. Find the third angle.
69° · 36° · Missing triangle angles
6. Two interior angles of a triangle measure 68° and 39°. Find the third angle.
68° · 39° · Missing triangle angles
7. Two interior angles of a triangle measure 67° and 57°. Find the third angle.
67° · 57° · Missing triangle angles
8. Two interior angles of a triangle measure 30° and 54°. Find the third angle.
30° · 54° · Missing triangle angles
9. Two interior angles of a triangle measure 54° and 31°. Find the third angle.
54° · 31° · Missing triangle angles
10. Two interior angles of a triangle measure 73° and 47°. Find the third angle.
73° · 47° · Missing triangle angles
11. Two interior angles of a triangle measure 77° and 31°. Find the third angle.
77° · 31° · Missing triangle angles
12. Two interior angles of a triangle measure 30° and 75°. Find the third angle.
30° · 75° · Missing triangle angles

12 questions · Seed 54 · 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 52° and 43°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (52° + 43°) = 85°.
2. Two interior angles of a triangle measure 29° and 38°. Find the third angle.
113°
Angles in a triangle sum to 180°: 180° − (29° + 38°) = 113°.
3. Two interior angles of a triangle measure 76° and 63°. Find the third angle.
41°
Angles in a triangle sum to 180°: 180° − (76° + 63°) = 41°.
4. Two interior angles of a triangle measure 29° and 45°. Find the third angle.
106°
Angles in a triangle sum to 180°: 180° − (29° + 45°) = 106°.
5. Two interior angles of a triangle measure 69° and 36°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (69° + 36°) = 75°.
6. Two interior angles of a triangle measure 68° and 39°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (68° + 39°) = 73°.
7. Two interior angles of a triangle measure 67° and 57°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (67° + 57°) = 56°.
8. Two interior angles of a triangle measure 30° and 54°. Find the third angle.
96°
Angles in a triangle sum to 180°: 180° − (30° + 54°) = 96°.
9. Two interior angles of a triangle measure 54° and 31°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (54° + 31°) = 95°.
10. Two interior angles of a triangle measure 73° and 47°. Find the third angle.
60°
Angles in a triangle sum to 180°: 180° − (73° + 47°) = 60°.
11. Two interior angles of a triangle measure 77° and 31°. Find the third angle.
72°
Angles in a triangle sum to 180°: 180° − (77° + 31°) = 72°.
12. Two interior angles of a triangle measure 30° and 75°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (30° + 75°) = 75°.

12 questions · Seed 54 · 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 52° and 43°. Find the third angle.
52° · 43° · Missing triangle angles
2. Two interior angles of a triangle measure 29° and 38°. Find the third angle.
29° · 38° · Missing triangle angles
3. Two interior angles of a triangle measure 76° and 63°. Find the third angle.
76° · 63° · Missing triangle angles
4. Two interior angles of a triangle measure 29° and 45°. Find the third angle.
29° · 45° · Missing triangle angles
5. Two interior angles of a triangle measure 69° and 36°. Find the third angle.
69° · 36° · Missing triangle angles
6. Two interior angles of a triangle measure 68° and 39°. Find the third angle.
68° · 39° · Missing triangle angles
7. Two interior angles of a triangle measure 67° and 57°. Find the third angle.
67° · 57° · Missing triangle angles
8. Two interior angles of a triangle measure 30° and 54°. Find the third angle.
30° · 54° · Missing triangle angles
9. Two interior angles of a triangle measure 54° and 31°. Find the third angle.
54° · 31° · Missing triangle angles
10. Two interior angles of a triangle measure 73° and 47°. Find the third angle.
73° · 47° · Missing triangle angles
11. Two interior angles of a triangle measure 77° and 31°. Find the third angle.
77° · 31° · Missing triangle angles
12. Two interior angles of a triangle measure 30° and 75°. Find the third angle.
30° · 75° · Missing triangle angles

12 questions · Seed 54 · 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 52° and 43°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (52° + 43°) = 85°.
2. Two interior angles of a triangle measure 29° and 38°. Find the third angle.
113°
Angles in a triangle sum to 180°: 180° − (29° + 38°) = 113°.
3. Two interior angles of a triangle measure 76° and 63°. Find the third angle.
41°
Angles in a triangle sum to 180°: 180° − (76° + 63°) = 41°.
4. Two interior angles of a triangle measure 29° and 45°. Find the third angle.
106°
Angles in a triangle sum to 180°: 180° − (29° + 45°) = 106°.
5. Two interior angles of a triangle measure 69° and 36°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (69° + 36°) = 75°.
6. Two interior angles of a triangle measure 68° and 39°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (68° + 39°) = 73°.
7. Two interior angles of a triangle measure 67° and 57°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (67° + 57°) = 56°.
8. Two interior angles of a triangle measure 30° and 54°. Find the third angle.
96°
Angles in a triangle sum to 180°: 180° − (30° + 54°) = 96°.
9. Two interior angles of a triangle measure 54° and 31°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (54° + 31°) = 95°.
10. Two interior angles of a triangle measure 73° and 47°. Find the third angle.
60°
Angles in a triangle sum to 180°: 180° − (73° + 47°) = 60°.
11. Two interior angles of a triangle measure 77° and 31°. Find the third angle.
72°
Angles in a triangle sum to 180°: 180° − (77° + 31°) = 72°.
12. Two interior angles of a triangle measure 30° and 75°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (30° + 75°) = 75°.

12 questions · Seed 54 · Age guidance, not curriculum certification