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 28° and 72°. Find the third angle.
28° · 72° · Missing triangle angles
2. Two interior angles of a triangle measure 73° and 42°. Find the third angle.
73° · 42° · Missing triangle angles
3. Two interior angles of a triangle measure 60° and 69°. Find the third angle.
60° · 69° · Missing triangle angles
4. Two interior angles of a triangle measure 54° and 57°. Find the third angle.
54° · 57° · Missing triangle angles
5. Two interior angles of a triangle measure 75° and 53°. Find the third angle.
75° · 53° · Missing triangle angles
6. Two interior angles of a triangle measure 81° and 71°. Find the third angle.
81° · 71° · Missing triangle angles
7. Two interior angles of a triangle measure 53° and 49°. Find the third angle.
53° · 49° · Missing triangle angles
8. Two interior angles of a triangle measure 25° and 48°. Find the third angle.
25° · 48° · Missing triangle angles
9. Two interior angles of a triangle measure 69° and 42°. Find the third angle.
69° · 42° · Missing triangle angles
10. Two interior angles of a triangle measure 57° and 46°. Find the third angle.
57° · 46° · Missing triangle angles
11. Two interior angles of a triangle measure 40° and 33°. Find the third angle.
40° · 33° · Missing triangle angles
12. Two interior angles of a triangle measure 82° and 62°. Find the third angle.
82° · 62° · Missing triangle angles

12 questions · Seed 61 · 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 28° and 72°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (28° + 72°) = 80°.
2. Two interior angles of a triangle measure 73° and 42°. Find the third angle.
65°
Angles in a triangle sum to 180°: 180° − (73° + 42°) = 65°.
3. Two interior angles of a triangle measure 60° and 69°. Find the third angle.
51°
Angles in a triangle sum to 180°: 180° − (60° + 69°) = 51°.
4. Two interior angles of a triangle measure 54° and 57°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (54° + 57°) = 69°.
5. Two interior angles of a triangle measure 75° and 53°. Find the third angle.
52°
Angles in a triangle sum to 180°: 180° − (75° + 53°) = 52°.
6. Two interior angles of a triangle measure 81° and 71°. Find the third angle.
28°
Angles in a triangle sum to 180°: 180° − (81° + 71°) = 28°.
7. Two interior angles of a triangle measure 53° and 49°. Find the third angle.
78°
Angles in a triangle sum to 180°: 180° − (53° + 49°) = 78°.
8. Two interior angles of a triangle measure 25° and 48°. Find the third angle.
107°
Angles in a triangle sum to 180°: 180° − (25° + 48°) = 107°.
9. Two interior angles of a triangle measure 69° and 42°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (69° + 42°) = 69°.
10. Two interior angles of a triangle measure 57° and 46°. Find the third angle.
77°
Angles in a triangle sum to 180°: 180° − (57° + 46°) = 77°.
11. Two interior angles of a triangle measure 40° and 33°. Find the third angle.
107°
Angles in a triangle sum to 180°: 180° − (40° + 33°) = 107°.
12. Two interior angles of a triangle measure 82° and 62°. Find the third angle.
36°
Angles in a triangle sum to 180°: 180° − (82° + 62°) = 36°.

12 questions · Seed 61 · 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 28° and 72°. Find the third angle.
28° · 72° · Missing triangle angles
2. Two interior angles of a triangle measure 73° and 42°. Find the third angle.
73° · 42° · Missing triangle angles
3. Two interior angles of a triangle measure 60° and 69°. Find the third angle.
60° · 69° · Missing triangle angles
4. Two interior angles of a triangle measure 54° and 57°. Find the third angle.
54° · 57° · Missing triangle angles
5. Two interior angles of a triangle measure 75° and 53°. Find the third angle.
75° · 53° · Missing triangle angles
6. Two interior angles of a triangle measure 81° and 71°. Find the third angle.
81° · 71° · Missing triangle angles
7. Two interior angles of a triangle measure 53° and 49°. Find the third angle.
53° · 49° · Missing triangle angles
8. Two interior angles of a triangle measure 25° and 48°. Find the third angle.
25° · 48° · Missing triangle angles
9. Two interior angles of a triangle measure 69° and 42°. Find the third angle.
69° · 42° · Missing triangle angles
10. Two interior angles of a triangle measure 57° and 46°. Find the third angle.
57° · 46° · Missing triangle angles
11. Two interior angles of a triangle measure 40° and 33°. Find the third angle.
40° · 33° · Missing triangle angles
12. Two interior angles of a triangle measure 82° and 62°. Find the third angle.
82° · 62° · Missing triangle angles

12 questions · Seed 61 · 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 28° and 72°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (28° + 72°) = 80°.
2. Two interior angles of a triangle measure 73° and 42°. Find the third angle.
65°
Angles in a triangle sum to 180°: 180° − (73° + 42°) = 65°.
3. Two interior angles of a triangle measure 60° and 69°. Find the third angle.
51°
Angles in a triangle sum to 180°: 180° − (60° + 69°) = 51°.
4. Two interior angles of a triangle measure 54° and 57°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (54° + 57°) = 69°.
5. Two interior angles of a triangle measure 75° and 53°. Find the third angle.
52°
Angles in a triangle sum to 180°: 180° − (75° + 53°) = 52°.
6. Two interior angles of a triangle measure 81° and 71°. Find the third angle.
28°
Angles in a triangle sum to 180°: 180° − (81° + 71°) = 28°.
7. Two interior angles of a triangle measure 53° and 49°. Find the third angle.
78°
Angles in a triangle sum to 180°: 180° − (53° + 49°) = 78°.
8. Two interior angles of a triangle measure 25° and 48°. Find the third angle.
107°
Angles in a triangle sum to 180°: 180° − (25° + 48°) = 107°.
9. Two interior angles of a triangle measure 69° and 42°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (69° + 42°) = 69°.
10. Two interior angles of a triangle measure 57° and 46°. Find the third angle.
77°
Angles in a triangle sum to 180°: 180° − (57° + 46°) = 77°.
11. Two interior angles of a triangle measure 40° and 33°. Find the third angle.
107°
Angles in a triangle sum to 180°: 180° − (40° + 33°) = 107°.
12. Two interior angles of a triangle measure 82° and 62°. Find the third angle.
36°
Angles in a triangle sum to 180°: 180° − (82° + 62°) = 36°.

12 questions · Seed 61 · Age guidance, not curriculum certification