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 75° and 65°. Find the third angle.
75° · 65° · Missing triangle angles
2. Two interior angles of a triangle measure 57° and 47°. Find the third angle.
57° · 47° · Missing triangle angles
3. Two interior angles of a triangle measure 83° and 41°. Find the third angle.
83° · 41° · Missing triangle angles
4. Two interior angles of a triangle measure 76° and 31°. Find the third angle.
76° · 31° · Missing triangle angles
5. Two interior angles of a triangle measure 84° and 27°. Find the third angle.
84° · 27° · Missing triangle angles
6. Two interior angles of a triangle measure 60° and 54°. Find the third angle.
60° · 54° · Missing triangle angles
7. Two interior angles of a triangle measure 40° and 46°. Find the third angle.
40° · 46° · Missing triangle angles
8. Two interior angles of a triangle measure 65° and 47°. Find the third angle.
65° · 47° · Missing triangle angles
9. Two interior angles of a triangle measure 55° and 71°. Find the third angle.
55° · 71° · Missing triangle angles
10. Two interior angles of a triangle measure 46° and 37°. Find the third angle.
46° · 37° · Missing triangle angles
11. Two interior angles of a triangle measure 46° and 58°. Find the third angle.
46° · 58° · Missing triangle angles
12. Two interior angles of a triangle measure 71° and 33°. Find the third angle.
71° · 33° · Missing triangle angles

12 questions · Seed 57 · 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 75° and 65°. Find the third angle.
40°
Angles in a triangle sum to 180°: 180° − (75° + 65°) = 40°.
2. Two interior angles of a triangle measure 57° and 47°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (57° + 47°) = 76°.
3. Two interior angles of a triangle measure 83° and 41°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (83° + 41°) = 56°.
4. Two interior angles of a triangle measure 76° and 31°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (76° + 31°) = 73°.
5. Two interior angles of a triangle measure 84° and 27°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (84° + 27°) = 69°.
6. Two interior angles of a triangle measure 60° and 54°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (60° + 54°) = 66°.
7. Two interior angles of a triangle measure 40° and 46°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (40° + 46°) = 94°.
8. Two interior angles of a triangle measure 65° and 47°. Find the third angle.
68°
Angles in a triangle sum to 180°: 180° − (65° + 47°) = 68°.
9. Two interior angles of a triangle measure 55° and 71°. Find the third angle.
54°
Angles in a triangle sum to 180°: 180° − (55° + 71°) = 54°.
10. Two interior angles of a triangle measure 46° and 37°. Find the third angle.
97°
Angles in a triangle sum to 180°: 180° − (46° + 37°) = 97°.
11. Two interior angles of a triangle measure 46° and 58°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (46° + 58°) = 76°.
12. Two interior angles of a triangle measure 71° and 33°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (71° + 33°) = 76°.

12 questions · Seed 57 · 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 75° and 65°. Find the third angle.
75° · 65° · Missing triangle angles
2. Two interior angles of a triangle measure 57° and 47°. Find the third angle.
57° · 47° · Missing triangle angles
3. Two interior angles of a triangle measure 83° and 41°. Find the third angle.
83° · 41° · Missing triangle angles
4. Two interior angles of a triangle measure 76° and 31°. Find the third angle.
76° · 31° · Missing triangle angles
5. Two interior angles of a triangle measure 84° and 27°. Find the third angle.
84° · 27° · Missing triangle angles
6. Two interior angles of a triangle measure 60° and 54°. Find the third angle.
60° · 54° · Missing triangle angles
7. Two interior angles of a triangle measure 40° and 46°. Find the third angle.
40° · 46° · Missing triangle angles
8. Two interior angles of a triangle measure 65° and 47°. Find the third angle.
65° · 47° · Missing triangle angles
9. Two interior angles of a triangle measure 55° and 71°. Find the third angle.
55° · 71° · Missing triangle angles
10. Two interior angles of a triangle measure 46° and 37°. Find the third angle.
46° · 37° · Missing triangle angles
11. Two interior angles of a triangle measure 46° and 58°. Find the third angle.
46° · 58° · Missing triangle angles
12. Two interior angles of a triangle measure 71° and 33°. Find the third angle.
71° · 33° · Missing triangle angles

12 questions · Seed 57 · 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 75° and 65°. Find the third angle.
40°
Angles in a triangle sum to 180°: 180° − (75° + 65°) = 40°.
2. Two interior angles of a triangle measure 57° and 47°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (57° + 47°) = 76°.
3. Two interior angles of a triangle measure 83° and 41°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (83° + 41°) = 56°.
4. Two interior angles of a triangle measure 76° and 31°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (76° + 31°) = 73°.
5. Two interior angles of a triangle measure 84° and 27°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (84° + 27°) = 69°.
6. Two interior angles of a triangle measure 60° and 54°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (60° + 54°) = 66°.
7. Two interior angles of a triangle measure 40° and 46°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (40° + 46°) = 94°.
8. Two interior angles of a triangle measure 65° and 47°. Find the third angle.
68°
Angles in a triangle sum to 180°: 180° − (65° + 47°) = 68°.
9. Two interior angles of a triangle measure 55° and 71°. Find the third angle.
54°
Angles in a triangle sum to 180°: 180° − (55° + 71°) = 54°.
10. Two interior angles of a triangle measure 46° and 37°. Find the third angle.
97°
Angles in a triangle sum to 180°: 180° − (46° + 37°) = 97°.
11. Two interior angles of a triangle measure 46° and 58°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (46° + 58°) = 76°.
12. Two interior angles of a triangle measure 71° and 33°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (71° + 33°) = 76°.

12 questions · Seed 57 · Age guidance, not curriculum certification