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 53° and 62°. Find the third angle.
53° · 62° · Missing triangle angles
2. Two interior angles of a triangle measure 75° and 55°. Find the third angle.
75° · 55° · Missing triangle angles
3. Two interior angles of a triangle measure 34° and 66°. Find the third angle.
34° · 66° · Missing triangle angles
4. Two interior angles of a triangle measure 40° and 39°. Find the third angle.
40° · 39° · Missing triangle angles
5. Two interior angles of a triangle measure 42° and 48°. Find the third angle.
42° · 48° · Missing triangle angles
6. Two interior angles of a triangle measure 65° and 30°. Find the third angle.
65° · 30° · Missing triangle angles
7. Two interior angles of a triangle measure 60° and 53°. Find the third angle.
60° · 53° · Missing triangle angles
8. Two interior angles of a triangle measure 66° and 50°. Find the third angle.
66° · 50° · Missing triangle angles
9. Two interior angles of a triangle measure 45° and 34°. Find the third angle.
45° · 34° · Missing triangle angles
10. Two interior angles of a triangle measure 78° and 69°. Find the third angle.
78° · 69° · Missing triangle angles
11. Two interior angles of a triangle measure 77° and 55°. Find the third angle.
77° · 55° · Missing triangle angles
12. Two interior angles of a triangle measure 40° and 26°. Find the third angle.
40° · 26° · Missing triangle angles

12 questions · Seed 104743 · 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 53° and 62°. Find the third angle.
65°
Angles in a triangle sum to 180°: 180° − (53° + 62°) = 65°.
2. Two interior angles of a triangle measure 75° and 55°. Find the third angle.
50°
Angles in a triangle sum to 180°: 180° − (75° + 55°) = 50°.
3. Two interior angles of a triangle measure 34° and 66°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (34° + 66°) = 80°.
4. 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°.
5. Two interior angles of a triangle measure 42° and 48°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (42° + 48°) = 90°.
6. Two interior angles of a triangle measure 65° and 30°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (65° + 30°) = 85°.
7. Two interior angles of a triangle measure 60° and 53°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (60° + 53°) = 67°.
8. Two interior angles of a triangle measure 66° and 50°. Find the third angle.
64°
Angles in a triangle sum to 180°: 180° − (66° + 50°) = 64°.
9. Two interior angles of a triangle measure 45° and 34°. Find the third angle.
101°
Angles in a triangle sum to 180°: 180° − (45° + 34°) = 101°.
10. Two interior angles of a triangle measure 78° and 69°. Find the third angle.
33°
Angles in a triangle sum to 180°: 180° − (78° + 69°) = 33°.
11. Two interior angles of a triangle measure 77° and 55°. Find the third angle.
48°
Angles in a triangle sum to 180°: 180° − (77° + 55°) = 48°.
12. Two interior angles of a triangle measure 40° and 26°. Find the third angle.
114°
Angles in a triangle sum to 180°: 180° − (40° + 26°) = 114°.

12 questions · Seed 104743 · 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 53° and 62°. Find the third angle.
53° · 62° · Missing triangle angles
2. Two interior angles of a triangle measure 75° and 55°. Find the third angle.
75° · 55° · Missing triangle angles
3. Two interior angles of a triangle measure 34° and 66°. Find the third angle.
34° · 66° · Missing triangle angles
4. Two interior angles of a triangle measure 40° and 39°. Find the third angle.
40° · 39° · Missing triangle angles
5. Two interior angles of a triangle measure 42° and 48°. Find the third angle.
42° · 48° · Missing triangle angles
6. Two interior angles of a triangle measure 65° and 30°. Find the third angle.
65° · 30° · Missing triangle angles
7. Two interior angles of a triangle measure 60° and 53°. Find the third angle.
60° · 53° · Missing triangle angles
8. Two interior angles of a triangle measure 66° and 50°. Find the third angle.
66° · 50° · Missing triangle angles
9. Two interior angles of a triangle measure 45° and 34°. Find the third angle.
45° · 34° · Missing triangle angles
10. Two interior angles of a triangle measure 78° and 69°. Find the third angle.
78° · 69° · Missing triangle angles
11. Two interior angles of a triangle measure 77° and 55°. Find the third angle.
77° · 55° · Missing triangle angles
12. Two interior angles of a triangle measure 40° and 26°. Find the third angle.
40° · 26° · Missing triangle angles

12 questions · Seed 104743 · 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 53° and 62°. Find the third angle.
65°
Angles in a triangle sum to 180°: 180° − (53° + 62°) = 65°.
2. Two interior angles of a triangle measure 75° and 55°. Find the third angle.
50°
Angles in a triangle sum to 180°: 180° − (75° + 55°) = 50°.
3. Two interior angles of a triangle measure 34° and 66°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (34° + 66°) = 80°.
4. 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°.
5. Two interior angles of a triangle measure 42° and 48°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (42° + 48°) = 90°.
6. Two interior angles of a triangle measure 65° and 30°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (65° + 30°) = 85°.
7. Two interior angles of a triangle measure 60° and 53°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (60° + 53°) = 67°.
8. Two interior angles of a triangle measure 66° and 50°. Find the third angle.
64°
Angles in a triangle sum to 180°: 180° − (66° + 50°) = 64°.
9. Two interior angles of a triangle measure 45° and 34°. Find the third angle.
101°
Angles in a triangle sum to 180°: 180° − (45° + 34°) = 101°.
10. Two interior angles of a triangle measure 78° and 69°. Find the third angle.
33°
Angles in a triangle sum to 180°: 180° − (78° + 69°) = 33°.
11. Two interior angles of a triangle measure 77° and 55°. Find the third angle.
48°
Angles in a triangle sum to 180°: 180° − (77° + 55°) = 48°.
12. Two interior angles of a triangle measure 40° and 26°. Find the third angle.
114°
Angles in a triangle sum to 180°: 180° − (40° + 26°) = 114°.

12 questions · Seed 104743 · Age guidance, not curriculum certification