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.

New variation Try interactive challenge Browse templates

Age band:Ages 10-12 Ages 13-15

Choose “Save as PDF” to keep a copy. Use your browser’s paper setting for A4 or Letter.

Worksheet StudioPractice pack · Ages 10-12

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 47° and 43°. Find the third angle.
47° · 43° · Missing triangle angles
2. Two interior angles of a triangle measure 42° and 64°. Find the third angle.
42° · 64° · Missing triangle angles
3. Two interior angles of a triangle measure 71° and 26°. Find the third angle.
71° · 26° · Missing triangle angles
4. Two interior angles of a triangle measure 49° and 69°. Find the third angle.
49° · 69° · Missing triangle angles
5. Two interior angles of a triangle measure 47° and 45°. Find the third angle.
47° · 45° · Missing triangle angles
6. Two interior angles of a triangle measure 47° and 41°. Find the third angle.
47° · 41° · Missing triangle angles
7. Two interior angles of a triangle measure 26° and 71°. Find the third angle.
26° · 71° · Missing triangle angles
8. Two interior angles of a triangle measure 45° and 37°. Find the third angle.
45° · 37° · Missing triangle angles
9. Two interior angles of a triangle measure 46° and 45°. Find the third angle.
46° · 45° · Missing triangle angles
10. Two interior angles of a triangle measure 54° and 67°. Find the third angle.
54° · 67° · Missing triangle angles
11. Two interior angles of a triangle measure 54° and 28°. Find the third angle.
54° · 28° · Missing triangle angles
12. Two interior angles of a triangle measure 28° and 60°. Find the third angle.
28° · 60° · Missing triangle angles

12 questions · Seed 68 · 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 47° and 43°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (47° + 43°) = 90°.
2. Two interior angles of a triangle measure 42° and 64°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (42° + 64°) = 74°.
3. Two interior angles of a triangle measure 71° and 26°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (71° + 26°) = 83°.
4. Two interior angles of a triangle measure 49° and 69°. Find the third angle.
62°
Angles in a triangle sum to 180°: 180° − (49° + 69°) = 62°.
5. Two interior angles of a triangle measure 47° and 45°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (47° + 45°) = 88°.
6. Two interior angles of a triangle measure 47° and 41°. Find the third angle.
92°
Angles in a triangle sum to 180°: 180° − (47° + 41°) = 92°.
7. Two interior angles of a triangle measure 26° and 71°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (26° + 71°) = 83°.
8. Two interior angles of a triangle measure 45° and 37°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (45° + 37°) = 98°.
9. Two interior angles of a triangle measure 46° and 45°. Find the third angle.
89°
Angles in a triangle sum to 180°: 180° − (46° + 45°) = 89°.
10. Two interior angles of a triangle measure 54° and 67°. Find the third angle.
59°
Angles in a triangle sum to 180°: 180° − (54° + 67°) = 59°.
11. Two interior angles of a triangle measure 54° and 28°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (54° + 28°) = 98°.
12. Two interior angles of a triangle measure 28° and 60°. Find the third angle.
92°
Angles in a triangle sum to 180°: 180° − (28° + 60°) = 92°.

12 questions · Seed 68 · 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 47° and 43°. Find the third angle.
47° · 43° · Missing triangle angles
2. Two interior angles of a triangle measure 42° and 64°. Find the third angle.
42° · 64° · Missing triangle angles
3. Two interior angles of a triangle measure 71° and 26°. Find the third angle.
71° · 26° · Missing triangle angles
4. Two interior angles of a triangle measure 49° and 69°. Find the third angle.
49° · 69° · Missing triangle angles
5. Two interior angles of a triangle measure 47° and 45°. Find the third angle.
47° · 45° · Missing triangle angles
6. Two interior angles of a triangle measure 47° and 41°. Find the third angle.
47° · 41° · Missing triangle angles
7. Two interior angles of a triangle measure 26° and 71°. Find the third angle.
26° · 71° · Missing triangle angles
8. Two interior angles of a triangle measure 45° and 37°. Find the third angle.
45° · 37° · Missing triangle angles
9. Two interior angles of a triangle measure 46° and 45°. Find the third angle.
46° · 45° · Missing triangle angles
10. Two interior angles of a triangle measure 54° and 67°. Find the third angle.
54° · 67° · Missing triangle angles
11. Two interior angles of a triangle measure 54° and 28°. Find the third angle.
54° · 28° · Missing triangle angles
12. Two interior angles of a triangle measure 28° and 60°. Find the third angle.
28° · 60° · Missing triangle angles

12 questions · Seed 68 · 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 47° and 43°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (47° + 43°) = 90°.
2. Two interior angles of a triangle measure 42° and 64°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (42° + 64°) = 74°.
3. Two interior angles of a triangle measure 71° and 26°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (71° + 26°) = 83°.
4. Two interior angles of a triangle measure 49° and 69°. Find the third angle.
62°
Angles in a triangle sum to 180°: 180° − (49° + 69°) = 62°.
5. Two interior angles of a triangle measure 47° and 45°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (47° + 45°) = 88°.
6. Two interior angles of a triangle measure 47° and 41°. Find the third angle.
92°
Angles in a triangle sum to 180°: 180° − (47° + 41°) = 92°.
7. Two interior angles of a triangle measure 26° and 71°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (26° + 71°) = 83°.
8. Two interior angles of a triangle measure 45° and 37°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (45° + 37°) = 98°.
9. Two interior angles of a triangle measure 46° and 45°. Find the third angle.
89°
Angles in a triangle sum to 180°: 180° − (46° + 45°) = 89°.
10. Two interior angles of a triangle measure 54° and 67°. Find the third angle.
59°
Angles in a triangle sum to 180°: 180° − (54° + 67°) = 59°.
11. Two interior angles of a triangle measure 54° and 28°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (54° + 28°) = 98°.
12. Two interior angles of a triangle measure 28° and 60°. Find the third angle.
92°
Angles in a triangle sum to 180°: 180° − (28° + 60°) = 92°.

12 questions · Seed 68 · Age guidance, not curriculum certification