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 74° and 30°. Find the third angle.
74° · 30° · Missing triangle angles
2. Two interior angles of a triangle measure 69° and 55°. Find the third angle.
69° · 55° · Missing triangle angles
3. Two interior angles of a triangle measure 61° and 38°. Find the third angle.
61° · 38° · Missing triangle angles
4. Two interior angles of a triangle measure 47° and 59°. Find the third angle.
47° · 59° · Missing triangle angles
5. Two interior angles of a triangle measure 62° and 64°. Find the third angle.
62° · 64° · Missing triangle angles
6. Two interior angles of a triangle measure 66° and 31°. Find the third angle.
66° · 31° · Missing triangle angles
7. Two interior angles of a triangle measure 51° and 44°. Find the third angle.
51° · 44° · Missing triangle angles
8. Two interior angles of a triangle measure 48° and 33°. Find the third angle.
48° · 33° · Missing triangle angles
9. Two interior angles of a triangle measure 35° and 63°. Find the third angle.
35° · 63° · Missing triangle angles
10. Two interior angles of a triangle measure 37° and 53°. Find the third angle.
37° · 53° · Missing triangle angles
11. Two interior angles of a triangle measure 73° and 28°. Find the third angle.
73° · 28° · Missing triangle angles
12. Two interior angles of a triangle measure 47° and 37°. Find the third angle.
47° · 37° · Missing triangle angles

12 questions · Seed 48 · 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 74° and 30°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (74° + 30°) = 76°.
2. Two interior angles of a triangle measure 69° and 55°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (69° + 55°) = 56°.
3. Two interior angles of a triangle measure 61° and 38°. Find the third angle.
81°
Angles in a triangle sum to 180°: 180° − (61° + 38°) = 81°.
4. Two interior angles of a triangle measure 47° and 59°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (47° + 59°) = 74°.
5. Two interior angles of a triangle measure 62° and 64°. Find the third angle.
54°
Angles in a triangle sum to 180°: 180° − (62° + 64°) = 54°.
6. Two interior angles of a triangle measure 66° and 31°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (66° + 31°) = 83°.
7. Two interior angles of a triangle measure 51° and 44°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (51° + 44°) = 85°.
8. Two interior angles of a triangle measure 48° and 33°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (48° + 33°) = 99°.
9. Two interior angles of a triangle measure 35° and 63°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (35° + 63°) = 82°.
10. Two interior angles of a triangle measure 37° and 53°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (37° + 53°) = 90°.
11. Two interior angles of a triangle measure 73° and 28°. Find the third angle.
79°
Angles in a triangle sum to 180°: 180° − (73° + 28°) = 79°.
12. Two interior angles of a triangle measure 47° and 37°. Find the third angle.
96°
Angles in a triangle sum to 180°: 180° − (47° + 37°) = 96°.

12 questions · Seed 48 · 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 74° and 30°. Find the third angle.
74° · 30° · Missing triangle angles
2. Two interior angles of a triangle measure 69° and 55°. Find the third angle.
69° · 55° · Missing triangle angles
3. Two interior angles of a triangle measure 61° and 38°. Find the third angle.
61° · 38° · Missing triangle angles
4. Two interior angles of a triangle measure 47° and 59°. Find the third angle.
47° · 59° · Missing triangle angles
5. Two interior angles of a triangle measure 62° and 64°. Find the third angle.
62° · 64° · Missing triangle angles
6. Two interior angles of a triangle measure 66° and 31°. Find the third angle.
66° · 31° · Missing triangle angles
7. Two interior angles of a triangle measure 51° and 44°. Find the third angle.
51° · 44° · Missing triangle angles
8. Two interior angles of a triangle measure 48° and 33°. Find the third angle.
48° · 33° · Missing triangle angles
9. Two interior angles of a triangle measure 35° and 63°. Find the third angle.
35° · 63° · Missing triangle angles
10. Two interior angles of a triangle measure 37° and 53°. Find the third angle.
37° · 53° · Missing triangle angles
11. Two interior angles of a triangle measure 73° and 28°. Find the third angle.
73° · 28° · Missing triangle angles
12. Two interior angles of a triangle measure 47° and 37°. Find the third angle.
47° · 37° · Missing triangle angles

12 questions · Seed 48 · 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 74° and 30°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (74° + 30°) = 76°.
2. Two interior angles of a triangle measure 69° and 55°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (69° + 55°) = 56°.
3. Two interior angles of a triangle measure 61° and 38°. Find the third angle.
81°
Angles in a triangle sum to 180°: 180° − (61° + 38°) = 81°.
4. Two interior angles of a triangle measure 47° and 59°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (47° + 59°) = 74°.
5. Two interior angles of a triangle measure 62° and 64°. Find the third angle.
54°
Angles in a triangle sum to 180°: 180° − (62° + 64°) = 54°.
6. Two interior angles of a triangle measure 66° and 31°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (66° + 31°) = 83°.
7. Two interior angles of a triangle measure 51° and 44°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (51° + 44°) = 85°.
8. Two interior angles of a triangle measure 48° and 33°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (48° + 33°) = 99°.
9. Two interior angles of a triangle measure 35° and 63°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (35° + 63°) = 82°.
10. Two interior angles of a triangle measure 37° and 53°. Find the third angle.
90°
Angles in a triangle sum to 180°: 180° − (37° + 53°) = 90°.
11. Two interior angles of a triangle measure 73° and 28°. Find the third angle.
79°
Angles in a triangle sum to 180°: 180° − (73° + 28°) = 79°.
12. Two interior angles of a triangle measure 47° and 37°. Find the third angle.
96°
Angles in a triangle sum to 180°: 180° − (47° + 37°) = 96°.

12 questions · Seed 48 · Age guidance, not curriculum certification