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 56° and 33°. Find the third angle.
56° · 33° · Missing triangle angles
2. Two interior angles of a triangle measure 39° and 57°. Find the third angle.
39° · 57° · Missing triangle angles
3. Two interior angles of a triangle measure 45° and 31°. Find the third angle.
45° · 31° · Missing triangle angles
4. Two interior angles of a triangle measure 77° and 74°. Find the third angle.
77° · 74° · Missing triangle angles
5. Two interior angles of a triangle measure 66° and 37°. Find the third angle.
66° · 37° · Missing triangle angles
6. Two interior angles of a triangle measure 25° and 41°. Find the third angle.
25° · 41° · Missing triangle angles
7. Two interior angles of a triangle measure 78° and 53°. Find the third angle.
78° · 53° · Missing triangle angles
8. Two interior angles of a triangle measure 61° and 61°. Find the third angle.
61° · 61° · Missing triangle angles
9. Two interior angles of a triangle measure 68° and 54°. Find the third angle.
68° · 54° · Missing triangle angles
10. Two interior angles of a triangle measure 37° and 29°. Find the third angle.
37° · 29° · Missing triangle angles
11. Two interior angles of a triangle measure 43° and 27°. Find the third angle.
43° · 27° · Missing triangle angles
12. Two interior angles of a triangle measure 52° and 61°. Find the third angle.
52° · 61° · Missing triangle angles

12 questions · Seed 104745 · 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 56° and 33°. Find the third angle.
91°
Angles in a triangle sum to 180°: 180° − (56° + 33°) = 91°.
2. Two interior angles of a triangle measure 39° and 57°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (39° + 57°) = 84°.
3. Two interior angles of a triangle measure 45° and 31°. Find the third angle.
104°
Angles in a triangle sum to 180°: 180° − (45° + 31°) = 104°.
4. Two interior angles of a triangle measure 77° and 74°. Find the third angle.
29°
Angles in a triangle sum to 180°: 180° − (77° + 74°) = 29°.
5. Two interior angles of a triangle measure 66° and 37°. Find the third angle.
77°
Angles in a triangle sum to 180°: 180° − (66° + 37°) = 77°.
6. Two interior angles of a triangle measure 25° and 41°. Find the third angle.
114°
Angles in a triangle sum to 180°: 180° − (25° + 41°) = 114°.
7. Two interior angles of a triangle measure 78° and 53°. Find the third angle.
49°
Angles in a triangle sum to 180°: 180° − (78° + 53°) = 49°.
8. Two interior angles of a triangle measure 61° and 61°. Find the third angle.
58°
Angles in a triangle sum to 180°: 180° − (61° + 61°) = 58°.
9. Two interior angles of a triangle measure 68° and 54°. Find the third angle.
58°
Angles in a triangle sum to 180°: 180° − (68° + 54°) = 58°.
10. Two interior angles of a triangle measure 37° and 29°. Find the third angle.
114°
Angles in a triangle sum to 180°: 180° − (37° + 29°) = 114°.
11. Two interior angles of a triangle measure 43° and 27°. Find the third angle.
110°
Angles in a triangle sum to 180°: 180° − (43° + 27°) = 110°.
12. Two interior angles of a triangle measure 52° and 61°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (52° + 61°) = 67°.

12 questions · Seed 104745 · 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 56° and 33°. Find the third angle.
56° · 33° · Missing triangle angles
2. Two interior angles of a triangle measure 39° and 57°. Find the third angle.
39° · 57° · Missing triangle angles
3. Two interior angles of a triangle measure 45° and 31°. Find the third angle.
45° · 31° · Missing triangle angles
4. Two interior angles of a triangle measure 77° and 74°. Find the third angle.
77° · 74° · Missing triangle angles
5. Two interior angles of a triangle measure 66° and 37°. Find the third angle.
66° · 37° · Missing triangle angles
6. Two interior angles of a triangle measure 25° and 41°. Find the third angle.
25° · 41° · Missing triangle angles
7. Two interior angles of a triangle measure 78° and 53°. Find the third angle.
78° · 53° · Missing triangle angles
8. Two interior angles of a triangle measure 61° and 61°. Find the third angle.
61° · 61° · Missing triangle angles
9. Two interior angles of a triangle measure 68° and 54°. Find the third angle.
68° · 54° · Missing triangle angles
10. Two interior angles of a triangle measure 37° and 29°. Find the third angle.
37° · 29° · Missing triangle angles
11. Two interior angles of a triangle measure 43° and 27°. Find the third angle.
43° · 27° · Missing triangle angles
12. Two interior angles of a triangle measure 52° and 61°. Find the third angle.
52° · 61° · Missing triangle angles

12 questions · Seed 104745 · 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 56° and 33°. Find the third angle.
91°
Angles in a triangle sum to 180°: 180° − (56° + 33°) = 91°.
2. Two interior angles of a triangle measure 39° and 57°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (39° + 57°) = 84°.
3. Two interior angles of a triangle measure 45° and 31°. Find the third angle.
104°
Angles in a triangle sum to 180°: 180° − (45° + 31°) = 104°.
4. Two interior angles of a triangle measure 77° and 74°. Find the third angle.
29°
Angles in a triangle sum to 180°: 180° − (77° + 74°) = 29°.
5. Two interior angles of a triangle measure 66° and 37°. Find the third angle.
77°
Angles in a triangle sum to 180°: 180° − (66° + 37°) = 77°.
6. Two interior angles of a triangle measure 25° and 41°. Find the third angle.
114°
Angles in a triangle sum to 180°: 180° − (25° + 41°) = 114°.
7. Two interior angles of a triangle measure 78° and 53°. Find the third angle.
49°
Angles in a triangle sum to 180°: 180° − (78° + 53°) = 49°.
8. Two interior angles of a triangle measure 61° and 61°. Find the third angle.
58°
Angles in a triangle sum to 180°: 180° − (61° + 61°) = 58°.
9. Two interior angles of a triangle measure 68° and 54°. Find the third angle.
58°
Angles in a triangle sum to 180°: 180° − (68° + 54°) = 58°.
10. Two interior angles of a triangle measure 37° and 29°. Find the third angle.
114°
Angles in a triangle sum to 180°: 180° − (37° + 29°) = 114°.
11. Two interior angles of a triangle measure 43° and 27°. Find the third angle.
110°
Angles in a triangle sum to 180°: 180° − (43° + 27°) = 110°.
12. Two interior angles of a triangle measure 52° and 61°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (52° + 61°) = 67°.

12 questions · Seed 104745 · Age guidance, not curriculum certification