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 75°. Find the third angle.
82° · 75° · Missing triangle angles
2. Two interior angles of a triangle measure 60° and 25°. Find the third angle.
60° · 25° · Missing triangle angles
3. Two interior angles of a triangle measure 82° and 68°. Find the third angle.
82° · 68° · Missing triangle angles
4. Two interior angles of a triangle measure 67° and 68°. Find the third angle.
67° · 68° · Missing triangle angles
5. Two interior angles of a triangle measure 73° and 25°. Find the third angle.
73° · 25° · Missing triangle angles
6. Two interior angles of a triangle measure 77° and 42°. Find the third angle.
77° · 42° · Missing triangle angles
7. Two interior angles of a triangle measure 29° and 53°. Find the third angle.
29° · 53° · Missing triangle angles
8. Two interior angles of a triangle measure 56° and 57°. Find the third angle.
56° · 57° · Missing triangle angles
9. Two interior angles of a triangle measure 71° and 48°. Find the third angle.
71° · 48° · Missing triangle angles
10. Two interior angles of a triangle measure 42° and 38°. Find the third angle.
42° · 38° · Missing triangle angles
11. Two interior angles of a triangle measure 55° and 32°. Find the third angle.
55° · 32° · Missing triangle angles
12. Two interior angles of a triangle measure 55° and 50°. Find the third angle.
55° · 50° · Missing triangle angles

12 questions · Seed 51 · 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 75°. Find the third angle.
23°
Angles in a triangle sum to 180°: 180° − (82° + 75°) = 23°.
2. Two interior angles of a triangle measure 60° and 25°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (60° + 25°) = 95°.
3. Two interior angles of a triangle measure 82° and 68°. Find the third angle.
30°
Angles in a triangle sum to 180°: 180° − (82° + 68°) = 30°.
4. Two interior angles of a triangle measure 67° and 68°. Find the third angle.
45°
Angles in a triangle sum to 180°: 180° − (67° + 68°) = 45°.
5. Two interior angles of a triangle measure 73° and 25°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (73° + 25°) = 82°.
6. Two interior angles of a triangle measure 77° and 42°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (77° + 42°) = 61°.
7. Two interior angles of a triangle measure 29° and 53°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (29° + 53°) = 98°.
8. Two interior angles of a triangle measure 56° and 57°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (56° + 57°) = 67°.
9. Two interior angles of a triangle measure 71° and 48°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (71° + 48°) = 61°.
10. Two interior angles of a triangle measure 42° and 38°. Find the third angle.
100°
Angles in a triangle sum to 180°: 180° − (42° + 38°) = 100°.
11. Two interior angles of a triangle measure 55° and 32°. Find the third angle.
93°
Angles in a triangle sum to 180°: 180° − (55° + 32°) = 93°.
12. Two interior angles of a triangle measure 55° and 50°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (55° + 50°) = 75°.

12 questions · Seed 51 · 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 75°. Find the third angle.
82° · 75° · Missing triangle angles
2. Two interior angles of a triangle measure 60° and 25°. Find the third angle.
60° · 25° · Missing triangle angles
3. Two interior angles of a triangle measure 82° and 68°. Find the third angle.
82° · 68° · Missing triangle angles
4. Two interior angles of a triangle measure 67° and 68°. Find the third angle.
67° · 68° · Missing triangle angles
5. Two interior angles of a triangle measure 73° and 25°. Find the third angle.
73° · 25° · Missing triangle angles
6. Two interior angles of a triangle measure 77° and 42°. Find the third angle.
77° · 42° · Missing triangle angles
7. Two interior angles of a triangle measure 29° and 53°. Find the third angle.
29° · 53° · Missing triangle angles
8. Two interior angles of a triangle measure 56° and 57°. Find the third angle.
56° · 57° · Missing triangle angles
9. Two interior angles of a triangle measure 71° and 48°. Find the third angle.
71° · 48° · Missing triangle angles
10. Two interior angles of a triangle measure 42° and 38°. Find the third angle.
42° · 38° · Missing triangle angles
11. Two interior angles of a triangle measure 55° and 32°. Find the third angle.
55° · 32° · Missing triangle angles
12. Two interior angles of a triangle measure 55° and 50°. Find the third angle.
55° · 50° · Missing triangle angles

12 questions · Seed 51 · 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 75°. Find the third angle.
23°
Angles in a triangle sum to 180°: 180° − (82° + 75°) = 23°.
2. Two interior angles of a triangle measure 60° and 25°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (60° + 25°) = 95°.
3. Two interior angles of a triangle measure 82° and 68°. Find the third angle.
30°
Angles in a triangle sum to 180°: 180° − (82° + 68°) = 30°.
4. Two interior angles of a triangle measure 67° and 68°. Find the third angle.
45°
Angles in a triangle sum to 180°: 180° − (67° + 68°) = 45°.
5. Two interior angles of a triangle measure 73° and 25°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (73° + 25°) = 82°.
6. Two interior angles of a triangle measure 77° and 42°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (77° + 42°) = 61°.
7. Two interior angles of a triangle measure 29° and 53°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (29° + 53°) = 98°.
8. Two interior angles of a triangle measure 56° and 57°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (56° + 57°) = 67°.
9. Two interior angles of a triangle measure 71° and 48°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (71° + 48°) = 61°.
10. Two interior angles of a triangle measure 42° and 38°. Find the third angle.
100°
Angles in a triangle sum to 180°: 180° − (42° + 38°) = 100°.
11. Two interior angles of a triangle measure 55° and 32°. Find the third angle.
93°
Angles in a triangle sum to 180°: 180° − (55° + 32°) = 93°.
12. Two interior angles of a triangle measure 55° and 50°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (55° + 50°) = 75°.

12 questions · Seed 51 · Age guidance, not curriculum certification