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 40° and 49°. Find the third angle.
40° · 49° · Missing triangle angles
2. Two interior angles of a triangle measure 69° and 34°. Find the third angle.
69° · 34° · Missing triangle angles
3. Two interior angles of a triangle measure 71° and 36°. Find the third angle.
71° · 36° · Missing triangle angles
4. Two interior angles of a triangle measure 37° and 64°. Find the third angle.
37° · 64° · Missing triangle angles
5. Two interior angles of a triangle measure 31° and 33°. Find the third angle.
31° · 33° · Missing triangle angles
6. Two interior angles of a triangle measure 45° and 47°. Find the third angle.
45° · 47° · Missing triangle angles
7. Two interior angles of a triangle measure 74° and 35°. Find the third angle.
74° · 35° · Missing triangle angles
8. Two interior angles of a triangle measure 54° and 52°. Find the third angle.
54° · 52° · Missing triangle angles
9. Two interior angles of a triangle measure 76° and 68°. Find the third angle.
76° · 68° · Missing triangle angles
10. Two interior angles of a triangle measure 35° and 51°. Find the third angle.
35° · 51° · Missing triangle angles
11. Two interior angles of a triangle measure 75° and 67°. Find the third angle.
75° · 67° · Missing triangle angles
12. Two interior angles of a triangle measure 54° and 59°. Find the third angle.
54° · 59° · Missing triangle angles

12 questions · Seed 59 · 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 40° and 49°. Find the third angle.
91°
Angles in a triangle sum to 180°: 180° − (40° + 49°) = 91°.
2. Two interior angles of a triangle measure 69° and 34°. Find the third angle.
77°
Angles in a triangle sum to 180°: 180° − (69° + 34°) = 77°.
3. Two interior angles of a triangle measure 71° and 36°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (71° + 36°) = 73°.
4. Two interior angles of a triangle measure 37° and 64°. Find the third angle.
79°
Angles in a triangle sum to 180°: 180° − (37° + 64°) = 79°.
5. Two interior angles of a triangle measure 31° and 33°. Find the third angle.
116°
Angles in a triangle sum to 180°: 180° − (31° + 33°) = 116°.
6. Two interior angles of a triangle measure 45° and 47°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (45° + 47°) = 88°.
7. Two interior angles of a triangle measure 74° and 35°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (74° + 35°) = 71°.
8. Two interior angles of a triangle measure 54° and 52°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (54° + 52°) = 74°.
9. Two interior angles of a triangle measure 76° and 68°. Find the third angle.
36°
Angles in a triangle sum to 180°: 180° − (76° + 68°) = 36°.
10. Two interior angles of a triangle measure 35° and 51°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (35° + 51°) = 94°.
11. Two interior angles of a triangle measure 75° and 67°. Find the third angle.
38°
Angles in a triangle sum to 180°: 180° − (75° + 67°) = 38°.
12. Two interior angles of a triangle measure 54° and 59°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (54° + 59°) = 67°.

12 questions · Seed 59 · 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 40° and 49°. Find the third angle.
40° · 49° · Missing triangle angles
2. Two interior angles of a triangle measure 69° and 34°. Find the third angle.
69° · 34° · Missing triangle angles
3. Two interior angles of a triangle measure 71° and 36°. Find the third angle.
71° · 36° · Missing triangle angles
4. Two interior angles of a triangle measure 37° and 64°. Find the third angle.
37° · 64° · Missing triangle angles
5. Two interior angles of a triangle measure 31° and 33°. Find the third angle.
31° · 33° · Missing triangle angles
6. Two interior angles of a triangle measure 45° and 47°. Find the third angle.
45° · 47° · Missing triangle angles
7. Two interior angles of a triangle measure 74° and 35°. Find the third angle.
74° · 35° · Missing triangle angles
8. Two interior angles of a triangle measure 54° and 52°. Find the third angle.
54° · 52° · Missing triangle angles
9. Two interior angles of a triangle measure 76° and 68°. Find the third angle.
76° · 68° · Missing triangle angles
10. Two interior angles of a triangle measure 35° and 51°. Find the third angle.
35° · 51° · Missing triangle angles
11. Two interior angles of a triangle measure 75° and 67°. Find the third angle.
75° · 67° · Missing triangle angles
12. Two interior angles of a triangle measure 54° and 59°. Find the third angle.
54° · 59° · Missing triangle angles

12 questions · Seed 59 · 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 40° and 49°. Find the third angle.
91°
Angles in a triangle sum to 180°: 180° − (40° + 49°) = 91°.
2. Two interior angles of a triangle measure 69° and 34°. Find the third angle.
77°
Angles in a triangle sum to 180°: 180° − (69° + 34°) = 77°.
3. Two interior angles of a triangle measure 71° and 36°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (71° + 36°) = 73°.
4. Two interior angles of a triangle measure 37° and 64°. Find the third angle.
79°
Angles in a triangle sum to 180°: 180° − (37° + 64°) = 79°.
5. Two interior angles of a triangle measure 31° and 33°. Find the third angle.
116°
Angles in a triangle sum to 180°: 180° − (31° + 33°) = 116°.
6. Two interior angles of a triangle measure 45° and 47°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (45° + 47°) = 88°.
7. Two interior angles of a triangle measure 74° and 35°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (74° + 35°) = 71°.
8. Two interior angles of a triangle measure 54° and 52°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (54° + 52°) = 74°.
9. Two interior angles of a triangle measure 76° and 68°. Find the third angle.
36°
Angles in a triangle sum to 180°: 180° − (76° + 68°) = 36°.
10. Two interior angles of a triangle measure 35° and 51°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (35° + 51°) = 94°.
11. Two interior angles of a triangle measure 75° and 67°. Find the third angle.
38°
Angles in a triangle sum to 180°: 180° − (75° + 67°) = 38°.
12. Two interior angles of a triangle measure 54° and 59°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (54° + 59°) = 67°.

12 questions · Seed 59 · Age guidance, not curriculum certification