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 25° and 75°. Find the third angle.
25° · 75° · Missing triangle angles
2. Two interior angles of a triangle measure 68° and 28°. Find the third angle.
68° · 28° · Missing triangle angles
3. Two interior angles of a triangle measure 65° and 43°. Find the third angle.
65° · 43° · Missing triangle angles
4. Two interior angles of a triangle measure 74° and 57°. Find the third angle.
74° · 57° · Missing triangle angles
5. Two interior angles of a triangle measure 57° and 71°. Find the third angle.
57° · 71° · Missing triangle angles
6. Two interior angles of a triangle measure 69° and 61°. Find the third angle.
69° · 61° · Missing triangle angles
7. Two interior angles of a triangle measure 82° and 51°. Find the third angle.
82° · 51° · Missing triangle angles
8. Two interior angles of a triangle measure 33° and 63°. Find the third angle.
33° · 63° · Missing triangle angles
9. Two interior angles of a triangle measure 76° and 54°. Find the third angle.
76° · 54° · Missing triangle angles
10. Two interior angles of a triangle measure 48° and 73°. Find the third angle.
48° · 73° · Missing triangle angles
11. Two interior angles of a triangle measure 80° and 67°. Find the third angle.
80° · 67° · Missing triangle angles
12. Two interior angles of a triangle measure 76° and 57°. Find the third angle.
76° · 57° · Missing triangle angles

12 questions · Seed 104735 · 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 25° and 75°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (25° + 75°) = 80°.
2. Two interior angles of a triangle measure 68° and 28°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (68° + 28°) = 84°.
3. Two interior angles of a triangle measure 65° and 43°. Find the third angle.
72°
Angles in a triangle sum to 180°: 180° − (65° + 43°) = 72°.
4. Two interior angles of a triangle measure 74° and 57°. Find the third angle.
49°
Angles in a triangle sum to 180°: 180° − (74° + 57°) = 49°.
5. Two interior angles of a triangle measure 57° and 71°. Find the third angle.
52°
Angles in a triangle sum to 180°: 180° − (57° + 71°) = 52°.
6. Two interior angles of a triangle measure 69° and 61°. Find the third angle.
50°
Angles in a triangle sum to 180°: 180° − (69° + 61°) = 50°.
7. Two interior angles of a triangle measure 82° and 51°. Find the third angle.
47°
Angles in a triangle sum to 180°: 180° − (82° + 51°) = 47°.
8. Two interior angles of a triangle measure 33° and 63°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (33° + 63°) = 84°.
9. Two interior angles of a triangle measure 76° and 54°. Find the third angle.
50°
Angles in a triangle sum to 180°: 180° − (76° + 54°) = 50°.
10. Two interior angles of a triangle measure 48° and 73°. Find the third angle.
59°
Angles in a triangle sum to 180°: 180° − (48° + 73°) = 59°.
11. Two interior angles of a triangle measure 80° and 67°. Find the third angle.
33°
Angles in a triangle sum to 180°: 180° − (80° + 67°) = 33°.
12. Two interior angles of a triangle measure 76° and 57°. Find the third angle.
47°
Angles in a triangle sum to 180°: 180° − (76° + 57°) = 47°.

12 questions · Seed 104735 · 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 25° and 75°. Find the third angle.
25° · 75° · Missing triangle angles
2. Two interior angles of a triangle measure 68° and 28°. Find the third angle.
68° · 28° · Missing triangle angles
3. Two interior angles of a triangle measure 65° and 43°. Find the third angle.
65° · 43° · Missing triangle angles
4. Two interior angles of a triangle measure 74° and 57°. Find the third angle.
74° · 57° · Missing triangle angles
5. Two interior angles of a triangle measure 57° and 71°. Find the third angle.
57° · 71° · Missing triangle angles
6. Two interior angles of a triangle measure 69° and 61°. Find the third angle.
69° · 61° · Missing triangle angles
7. Two interior angles of a triangle measure 82° and 51°. Find the third angle.
82° · 51° · Missing triangle angles
8. Two interior angles of a triangle measure 33° and 63°. Find the third angle.
33° · 63° · Missing triangle angles
9. Two interior angles of a triangle measure 76° and 54°. Find the third angle.
76° · 54° · Missing triangle angles
10. Two interior angles of a triangle measure 48° and 73°. Find the third angle.
48° · 73° · Missing triangle angles
11. Two interior angles of a triangle measure 80° and 67°. Find the third angle.
80° · 67° · Missing triangle angles
12. Two interior angles of a triangle measure 76° and 57°. Find the third angle.
76° · 57° · Missing triangle angles

12 questions · Seed 104735 · 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 25° and 75°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (25° + 75°) = 80°.
2. Two interior angles of a triangle measure 68° and 28°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (68° + 28°) = 84°.
3. Two interior angles of a triangle measure 65° and 43°. Find the third angle.
72°
Angles in a triangle sum to 180°: 180° − (65° + 43°) = 72°.
4. Two interior angles of a triangle measure 74° and 57°. Find the third angle.
49°
Angles in a triangle sum to 180°: 180° − (74° + 57°) = 49°.
5. Two interior angles of a triangle measure 57° and 71°. Find the third angle.
52°
Angles in a triangle sum to 180°: 180° − (57° + 71°) = 52°.
6. Two interior angles of a triangle measure 69° and 61°. Find the third angle.
50°
Angles in a triangle sum to 180°: 180° − (69° + 61°) = 50°.
7. Two interior angles of a triangle measure 82° and 51°. Find the third angle.
47°
Angles in a triangle sum to 180°: 180° − (82° + 51°) = 47°.
8. Two interior angles of a triangle measure 33° and 63°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (33° + 63°) = 84°.
9. Two interior angles of a triangle measure 76° and 54°. Find the third angle.
50°
Angles in a triangle sum to 180°: 180° − (76° + 54°) = 50°.
10. Two interior angles of a triangle measure 48° and 73°. Find the third angle.
59°
Angles in a triangle sum to 180°: 180° − (48° + 73°) = 59°.
11. Two interior angles of a triangle measure 80° and 67°. Find the third angle.
33°
Angles in a triangle sum to 180°: 180° − (80° + 67°) = 33°.
12. Two interior angles of a triangle measure 76° and 57°. Find the third angle.
47°
Angles in a triangle sum to 180°: 180° − (76° + 57°) = 47°.

12 questions · Seed 104735 · Age guidance, not curriculum certification