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 71° and 71°. Find the third angle.
71° · 71° · Missing triangle angles
2. Two interior angles of a triangle measure 31° and 41°. Find the third angle.
31° · 41° · Missing triangle angles
3. Two interior angles of a triangle measure 63° and 45°. Find the third angle.
63° · 45° · Missing triangle angles
4. Two interior angles of a triangle measure 69° and 45°. Find the third angle.
69° · 45° · Missing triangle angles
5. Two interior angles of a triangle measure 55° and 41°. Find the third angle.
55° · 41° · Missing triangle angles
6. Two interior angles of a triangle measure 27° and 52°. Find the third angle.
27° · 52° · Missing triangle angles
7. Two interior angles of a triangle measure 33° and 69°. Find the third angle.
33° · 69° · Missing triangle angles
8. Two interior angles of a triangle measure 46° and 48°. Find the third angle.
46° · 48° · Missing triangle angles
9. Two interior angles of a triangle measure 66° and 68°. Find the third angle.
66° · 68° · Missing triangle angles
10. Two interior angles of a triangle measure 84° and 67°. Find the third angle.
84° · 67° · Missing triangle angles
11. Two interior angles of a triangle measure 60° and 56°. Find the third angle.
60° · 56° · Missing triangle angles
12. Two interior angles of a triangle measure 32° and 35°. Find the third angle.
32° · 35° · Missing triangle angles

12 questions · Seed 104733 · 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 71° and 71°. Find the third angle.
38°
Angles in a triangle sum to 180°: 180° − (71° + 71°) = 38°.
2. Two interior angles of a triangle measure 31° and 41°. Find the third angle.
108°
Angles in a triangle sum to 180°: 180° − (31° + 41°) = 108°.
3. Two interior angles of a triangle measure 63° and 45°. Find the third angle.
72°
Angles in a triangle sum to 180°: 180° − (63° + 45°) = 72°.
4. Two interior angles of a triangle measure 69° and 45°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (69° + 45°) = 66°.
5. Two interior angles of a triangle measure 55° and 41°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (55° + 41°) = 84°.
6. Two interior angles of a triangle measure 27° and 52°. Find the third angle.
101°
Angles in a triangle sum to 180°: 180° − (27° + 52°) = 101°.
7. Two interior angles of a triangle measure 33° and 69°. Find the third angle.
78°
Angles in a triangle sum to 180°: 180° − (33° + 69°) = 78°.
8. Two interior angles of a triangle measure 46° and 48°. Find the third angle.
86°
Angles in a triangle sum to 180°: 180° − (46° + 48°) = 86°.
9. Two interior angles of a triangle measure 66° and 68°. Find the third angle.
46°
Angles in a triangle sum to 180°: 180° − (66° + 68°) = 46°.
10. Two interior angles of a triangle measure 84° and 67°. Find the third angle.
29°
Angles in a triangle sum to 180°: 180° − (84° + 67°) = 29°.
11. Two interior angles of a triangle measure 60° and 56°. Find the third angle.
64°
Angles in a triangle sum to 180°: 180° − (60° + 56°) = 64°.
12. Two interior angles of a triangle measure 32° and 35°. Find the third angle.
113°
Angles in a triangle sum to 180°: 180° − (32° + 35°) = 113°.

12 questions · Seed 104733 · 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 71° and 71°. Find the third angle.
71° · 71° · Missing triangle angles
2. Two interior angles of a triangle measure 31° and 41°. Find the third angle.
31° · 41° · Missing triangle angles
3. Two interior angles of a triangle measure 63° and 45°. Find the third angle.
63° · 45° · Missing triangle angles
4. Two interior angles of a triangle measure 69° and 45°. Find the third angle.
69° · 45° · Missing triangle angles
5. Two interior angles of a triangle measure 55° and 41°. Find the third angle.
55° · 41° · Missing triangle angles
6. Two interior angles of a triangle measure 27° and 52°. Find the third angle.
27° · 52° · Missing triangle angles
7. Two interior angles of a triangle measure 33° and 69°. Find the third angle.
33° · 69° · Missing triangle angles
8. Two interior angles of a triangle measure 46° and 48°. Find the third angle.
46° · 48° · Missing triangle angles
9. Two interior angles of a triangle measure 66° and 68°. Find the third angle.
66° · 68° · Missing triangle angles
10. Two interior angles of a triangle measure 84° and 67°. Find the third angle.
84° · 67° · Missing triangle angles
11. Two interior angles of a triangle measure 60° and 56°. Find the third angle.
60° · 56° · Missing triangle angles
12. Two interior angles of a triangle measure 32° and 35°. Find the third angle.
32° · 35° · Missing triangle angles

12 questions · Seed 104733 · 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 71° and 71°. Find the third angle.
38°
Angles in a triangle sum to 180°: 180° − (71° + 71°) = 38°.
2. Two interior angles of a triangle measure 31° and 41°. Find the third angle.
108°
Angles in a triangle sum to 180°: 180° − (31° + 41°) = 108°.
3. Two interior angles of a triangle measure 63° and 45°. Find the third angle.
72°
Angles in a triangle sum to 180°: 180° − (63° + 45°) = 72°.
4. Two interior angles of a triangle measure 69° and 45°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (69° + 45°) = 66°.
5. Two interior angles of a triangle measure 55° and 41°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (55° + 41°) = 84°.
6. Two interior angles of a triangle measure 27° and 52°. Find the third angle.
101°
Angles in a triangle sum to 180°: 180° − (27° + 52°) = 101°.
7. Two interior angles of a triangle measure 33° and 69°. Find the third angle.
78°
Angles in a triangle sum to 180°: 180° − (33° + 69°) = 78°.
8. Two interior angles of a triangle measure 46° and 48°. Find the third angle.
86°
Angles in a triangle sum to 180°: 180° − (46° + 48°) = 86°.
9. Two interior angles of a triangle measure 66° and 68°. Find the third angle.
46°
Angles in a triangle sum to 180°: 180° − (66° + 68°) = 46°.
10. Two interior angles of a triangle measure 84° and 67°. Find the third angle.
29°
Angles in a triangle sum to 180°: 180° − (84° + 67°) = 29°.
11. Two interior angles of a triangle measure 60° and 56°. Find the third angle.
64°
Angles in a triangle sum to 180°: 180° − (60° + 56°) = 64°.
12. Two interior angles of a triangle measure 32° and 35°. Find the third angle.
113°
Angles in a triangle sum to 180°: 180° − (32° + 35°) = 113°.

12 questions · Seed 104733 · Age guidance, not curriculum certification