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 66° and 40°. Find the third angle.
66° · 40° · Missing triangle angles
2. Two interior angles of a triangle measure 62° and 28°. Find the third angle.
62° · 28° · Missing triangle angles
3. Two interior angles of a triangle measure 59° and 59°. Find the third angle.
59° · 59° · Missing triangle angles
4. Two interior angles of a triangle measure 44° and 33°. Find the third angle.
44° · 33° · Missing triangle angles
5. Two interior angles of a triangle measure 58° and 70°. Find the third angle.
58° · 70° · Missing triangle angles
6. Two interior angles of a triangle measure 28° and 39°. Find the third angle.
28° · 39° · Missing triangle angles
7. Two interior angles of a triangle measure 66° and 41°. Find the third angle.
66° · 41° · Missing triangle angles
8. Two interior angles of a triangle measure 62° and 27°. Find the third angle.
62° · 27° · Missing triangle angles
9. Two interior angles of a triangle measure 54° and 53°. Find the third angle.
54° · 53° · Missing triangle angles
10. Two interior angles of a triangle measure 72° and 30°. Find the third angle.
72° · 30° · Missing triangle angles
11. Two interior angles of a triangle measure 40° and 59°. Find the third angle.
40° · 59° · Missing triangle angles
12. Two interior angles of a triangle measure 41° and 31°. Find the third angle.
41° · 31° · Missing triangle angles

12 questions · Seed 47 · 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 66° and 40°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (66° + 40°) = 74°.
2. Two interior angles of a triangle measure 62° and 28°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (62° + 28°) = 90°.
3. Two interior angles of a triangle measure 59° and 59°. Find the third angle.
62°
Angles in a triangle sum to 180°: 180° − (59° + 59°) = 62°.
4. Two interior angles of a triangle measure 44° and 33°. Find the third angle.
103°
Angles in a triangle sum to 180°: 180° − (44° + 33°) = 103°.
5. Two interior angles of a triangle measure 58° and 70°. Find the third angle.
52°
Angles in a triangle sum to 180°: 180° − (58° + 70°) = 52°.
6. Two interior angles of a triangle measure 28° and 39°. Find the third angle.
113°
Angles in a triangle sum to 180°: 180° − (28° + 39°) = 113°.
7. Two interior angles of a triangle measure 66° and 41°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (66° + 41°) = 73°.
8. Two interior angles of a triangle measure 62° and 27°. Find the third angle.
91°
Angles in a triangle sum to 180°: 180° − (62° + 27°) = 91°.
9. Two interior angles of a triangle measure 54° and 53°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (54° + 53°) = 73°.
10. Two interior angles of a triangle measure 72° and 30°. Find the third angle.
78°
Angles in a triangle sum to 180°: 180° − (72° + 30°) = 78°.
11. Two interior angles of a triangle measure 40° and 59°. Find the third angle.
81°
Angles in a triangle sum to 180°: 180° − (40° + 59°) = 81°.
12. Two interior angles of a triangle measure 41° and 31°. Find the third angle.
108°
Angles in a triangle sum to 180°: 180° − (41° + 31°) = 108°.

12 questions · Seed 47 · 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 66° and 40°. Find the third angle.
66° · 40° · Missing triangle angles
2. Two interior angles of a triangle measure 62° and 28°. Find the third angle.
62° · 28° · Missing triangle angles
3. Two interior angles of a triangle measure 59° and 59°. Find the third angle.
59° · 59° · Missing triangle angles
4. Two interior angles of a triangle measure 44° and 33°. Find the third angle.
44° · 33° · Missing triangle angles
5. Two interior angles of a triangle measure 58° and 70°. Find the third angle.
58° · 70° · Missing triangle angles
6. Two interior angles of a triangle measure 28° and 39°. Find the third angle.
28° · 39° · Missing triangle angles
7. Two interior angles of a triangle measure 66° and 41°. Find the third angle.
66° · 41° · Missing triangle angles
8. Two interior angles of a triangle measure 62° and 27°. Find the third angle.
62° · 27° · Missing triangle angles
9. Two interior angles of a triangle measure 54° and 53°. Find the third angle.
54° · 53° · Missing triangle angles
10. Two interior angles of a triangle measure 72° and 30°. Find the third angle.
72° · 30° · Missing triangle angles
11. Two interior angles of a triangle measure 40° and 59°. Find the third angle.
40° · 59° · Missing triangle angles
12. Two interior angles of a triangle measure 41° and 31°. Find the third angle.
41° · 31° · Missing triangle angles

12 questions · Seed 47 · 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 66° and 40°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (66° + 40°) = 74°.
2. Two interior angles of a triangle measure 62° and 28°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (62° + 28°) = 90°.
3. Two interior angles of a triangle measure 59° and 59°. Find the third angle.
62°
Angles in a triangle sum to 180°: 180° − (59° + 59°) = 62°.
4. Two interior angles of a triangle measure 44° and 33°. Find the third angle.
103°
Angles in a triangle sum to 180°: 180° − (44° + 33°) = 103°.
5. Two interior angles of a triangle measure 58° and 70°. Find the third angle.
52°
Angles in a triangle sum to 180°: 180° − (58° + 70°) = 52°.
6. Two interior angles of a triangle measure 28° and 39°. Find the third angle.
113°
Angles in a triangle sum to 180°: 180° − (28° + 39°) = 113°.
7. Two interior angles of a triangle measure 66° and 41°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (66° + 41°) = 73°.
8. Two interior angles of a triangle measure 62° and 27°. Find the third angle.
91°
Angles in a triangle sum to 180°: 180° − (62° + 27°) = 91°.
9. Two interior angles of a triangle measure 54° and 53°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (54° + 53°) = 73°.
10. Two interior angles of a triangle measure 72° and 30°. Find the third angle.
78°
Angles in a triangle sum to 180°: 180° − (72° + 30°) = 78°.
11. Two interior angles of a triangle measure 40° and 59°. Find the third angle.
81°
Angles in a triangle sum to 180°: 180° − (40° + 59°) = 81°.
12. Two interior angles of a triangle measure 41° and 31°. Find the third angle.
108°
Angles in a triangle sum to 180°: 180° − (41° + 31°) = 108°.

12 questions · Seed 47 · Age guidance, not curriculum certification