ReasoningAges 13-15

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 13-15

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 72° and 51°. Find the third angle.
72° · 51° · Missing triangle angles
2. Two interior angles of a triangle measure 38° and 45°. Find the third angle.
38° · 45° · Missing triangle angles
3. Two interior angles of a triangle measure 68° and 45°. Find the third angle.
68° · 45° · Missing triangle angles
4. Two interior angles of a triangle measure 35° and 43°. Find the third angle.
35° · 43° · Missing triangle angles
5. Two interior angles of a triangle measure 78° and 44°. Find the third angle.
78° · 44° · Missing triangle angles
6. Two interior angles of a triangle measure 40° and 39°. Find the third angle.
40° · 39° · Missing triangle angles
7. Two interior angles of a triangle measure 33° and 43°. Find the third angle.
33° · 43° · Missing triangle angles
8. Two interior angles of a triangle measure 43° and 62°. Find the third angle.
43° · 62° · Missing triangle angles
9. Two interior angles of a triangle measure 41° and 45°. Find the third angle.
41° · 45° · Missing triangle angles
10. Two interior angles of a triangle measure 61° and 39°. Find the third angle.
61° · 39° · Missing triangle angles
11. Two interior angles of a triangle measure 42° and 28°. Find the third angle.
42° · 28° · Missing triangle angles
12. Two interior angles of a triangle measure 40° and 53°. Find the third angle.
40° · 53° · Missing triangle angles

12 questions · Seed 104739 · Age guidance, not curriculum certification

View the answer key

Worksheet StudioAnswer key · Ages 13-15

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 72° and 51°. Find the third angle.
57°
Angles in a triangle sum to 180°: 180° − (72° + 51°) = 57°.
2. Two interior angles of a triangle measure 38° and 45°. Find the third angle.
97°
Angles in a triangle sum to 180°: 180° − (38° + 45°) = 97°.
3. Two interior angles of a triangle measure 68° and 45°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (68° + 45°) = 67°.
4. Two interior angles of a triangle measure 35° and 43°. Find the third angle.
102°
Angles in a triangle sum to 180°: 180° − (35° + 43°) = 102°.
5. Two interior angles of a triangle measure 78° and 44°. Find the third angle.
58°
Angles in a triangle sum to 180°: 180° − (78° + 44°) = 58°.
6. Two interior angles of a triangle measure 40° and 39°. Find the third angle.
101°
Angles in a triangle sum to 180°: 180° − (40° + 39°) = 101°.
7. Two interior angles of a triangle measure 33° and 43°. Find the third angle.
104°
Angles in a triangle sum to 180°: 180° − (33° + 43°) = 104°.
8. Two interior angles of a triangle measure 43° and 62°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (43° + 62°) = 75°.
9. Two interior angles of a triangle measure 41° and 45°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (41° + 45°) = 94°.
10. Two interior angles of a triangle measure 61° and 39°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (61° + 39°) = 80°.
11. Two interior angles of a triangle measure 42° and 28°. Find the third angle.
110°
Angles in a triangle sum to 180°: 180° − (42° + 28°) = 110°.
12. Two interior angles of a triangle measure 40° and 53°. Find the third angle.
87°
Angles in a triangle sum to 180°: 180° − (40° + 53°) = 87°.

12 questions · Seed 104739 · Age guidance, not curriculum certification

Worksheet StudioPractice pack · Ages 13-15

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 72° and 51°. Find the third angle.
72° · 51° · Missing triangle angles
2. Two interior angles of a triangle measure 38° and 45°. Find the third angle.
38° · 45° · Missing triangle angles
3. Two interior angles of a triangle measure 68° and 45°. Find the third angle.
68° · 45° · Missing triangle angles
4. Two interior angles of a triangle measure 35° and 43°. Find the third angle.
35° · 43° · Missing triangle angles
5. Two interior angles of a triangle measure 78° and 44°. Find the third angle.
78° · 44° · Missing triangle angles
6. Two interior angles of a triangle measure 40° and 39°. Find the third angle.
40° · 39° · Missing triangle angles
7. Two interior angles of a triangle measure 33° and 43°. Find the third angle.
33° · 43° · Missing triangle angles
8. Two interior angles of a triangle measure 43° and 62°. Find the third angle.
43° · 62° · Missing triangle angles
9. Two interior angles of a triangle measure 41° and 45°. Find the third angle.
41° · 45° · Missing triangle angles
10. Two interior angles of a triangle measure 61° and 39°. Find the third angle.
61° · 39° · Missing triangle angles
11. Two interior angles of a triangle measure 42° and 28°. Find the third angle.
42° · 28° · Missing triangle angles
12. Two interior angles of a triangle measure 40° and 53°. Find the third angle.
40° · 53° · Missing triangle angles

12 questions · Seed 104739 · Age guidance, not curriculum certification

Worksheet StudioAnswer key · Ages 13-15

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 72° and 51°. Find the third angle.
57°
Angles in a triangle sum to 180°: 180° − (72° + 51°) = 57°.
2. Two interior angles of a triangle measure 38° and 45°. Find the third angle.
97°
Angles in a triangle sum to 180°: 180° − (38° + 45°) = 97°.
3. Two interior angles of a triangle measure 68° and 45°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (68° + 45°) = 67°.
4. Two interior angles of a triangle measure 35° and 43°. Find the third angle.
102°
Angles in a triangle sum to 180°: 180° − (35° + 43°) = 102°.
5. Two interior angles of a triangle measure 78° and 44°. Find the third angle.
58°
Angles in a triangle sum to 180°: 180° − (78° + 44°) = 58°.
6. Two interior angles of a triangle measure 40° and 39°. Find the third angle.
101°
Angles in a triangle sum to 180°: 180° − (40° + 39°) = 101°.
7. Two interior angles of a triangle measure 33° and 43°. Find the third angle.
104°
Angles in a triangle sum to 180°: 180° − (33° + 43°) = 104°.
8. Two interior angles of a triangle measure 43° and 62°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (43° + 62°) = 75°.
9. Two interior angles of a triangle measure 41° and 45°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (41° + 45°) = 94°.
10. Two interior angles of a triangle measure 61° and 39°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (61° + 39°) = 80°.
11. Two interior angles of a triangle measure 42° and 28°. Find the third angle.
110°
Angles in a triangle sum to 180°: 180° − (42° + 28°) = 110°.
12. Two interior angles of a triangle measure 40° and 53°. Find the third angle.
87°
Angles in a triangle sum to 180°: 180° − (40° + 53°) = 87°.

12 questions · Seed 104739 · Age guidance, not curriculum certification