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 26° and 59°. Find the third angle.
26° · 59° · Missing triangle angles
2. Two interior angles of a triangle measure 65° and 27°. Find the third angle.
65° · 27° · Missing triangle angles
3. Two interior angles of a triangle measure 33° and 52°. Find the third angle.
33° · 52° · Missing triangle angles
4. Two interior angles of a triangle measure 38° and 71°. Find the third angle.
38° · 71° · Missing triangle angles
5. Two interior angles of a triangle measure 29° and 66°. Find the third angle.
29° · 66° · Missing triangle angles
6. Two interior angles of a triangle measure 27° and 72°. Find the third angle.
27° · 72° · Missing triangle angles
7. Two interior angles of a triangle measure 68° and 56°. Find the third angle.
68° · 56° · Missing triangle angles
8. Two interior angles of a triangle measure 66° and 75°. Find the third angle.
66° · 75° · Missing triangle angles
9. Two interior angles of a triangle measure 42° and 25°. Find the third angle.
42° · 25° · Missing triangle angles
10. Two interior angles of a triangle measure 40° and 42°. Find the third angle.
40° · 42° · Missing triangle angles
11. Two interior angles of a triangle measure 84° and 54°. Find the third angle.
84° · 54° · Missing triangle angles
12. Two interior angles of a triangle measure 55° and 40°. Find the third angle.
55° · 40° · Missing triangle angles

12 questions · Seed 104737 · 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 26° and 59°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (26° + 59°) = 95°.
2. Two interior angles of a triangle measure 65° and 27°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (65° + 27°) = 88°.
3. Two interior angles of a triangle measure 33° and 52°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (33° + 52°) = 95°.
4. Two interior angles of a triangle measure 38° and 71°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (38° + 71°) = 71°.
5. Two interior angles of a triangle measure 29° and 66°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (29° + 66°) = 85°.
6. Two interior angles of a triangle measure 27° and 72°. Find the third angle.
81°
Angles in a triangle sum to 180°: 180° − (27° + 72°) = 81°.
7. Two interior angles of a triangle measure 68° and 56°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (68° + 56°) = 56°.
8. Two interior angles of a triangle measure 66° and 75°. Find the third angle.
39°
Angles in a triangle sum to 180°: 180° − (66° + 75°) = 39°.
9. Two interior angles of a triangle measure 42° and 25°. Find the third angle.
113°
Angles in a triangle sum to 180°: 180° − (42° + 25°) = 113°.
10. Two interior angles of a triangle measure 40° and 42°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (40° + 42°) = 98°.
11. Two interior angles of a triangle measure 84° and 54°. Find the third angle.
42°
Angles in a triangle sum to 180°: 180° − (84° + 54°) = 42°.
12. Two interior angles of a triangle measure 55° and 40°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (55° + 40°) = 85°.

12 questions · Seed 104737 · 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 26° and 59°. Find the third angle.
26° · 59° · Missing triangle angles
2. Two interior angles of a triangle measure 65° and 27°. Find the third angle.
65° · 27° · Missing triangle angles
3. Two interior angles of a triangle measure 33° and 52°. Find the third angle.
33° · 52° · Missing triangle angles
4. Two interior angles of a triangle measure 38° and 71°. Find the third angle.
38° · 71° · Missing triangle angles
5. Two interior angles of a triangle measure 29° and 66°. Find the third angle.
29° · 66° · Missing triangle angles
6. Two interior angles of a triangle measure 27° and 72°. Find the third angle.
27° · 72° · Missing triangle angles
7. Two interior angles of a triangle measure 68° and 56°. Find the third angle.
68° · 56° · Missing triangle angles
8. Two interior angles of a triangle measure 66° and 75°. Find the third angle.
66° · 75° · Missing triangle angles
9. Two interior angles of a triangle measure 42° and 25°. Find the third angle.
42° · 25° · Missing triangle angles
10. Two interior angles of a triangle measure 40° and 42°. Find the third angle.
40° · 42° · Missing triangle angles
11. Two interior angles of a triangle measure 84° and 54°. Find the third angle.
84° · 54° · Missing triangle angles
12. Two interior angles of a triangle measure 55° and 40°. Find the third angle.
55° · 40° · Missing triangle angles

12 questions · Seed 104737 · 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 26° and 59°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (26° + 59°) = 95°.
2. Two interior angles of a triangle measure 65° and 27°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (65° + 27°) = 88°.
3. Two interior angles of a triangle measure 33° and 52°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (33° + 52°) = 95°.
4. Two interior angles of a triangle measure 38° and 71°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (38° + 71°) = 71°.
5. Two interior angles of a triangle measure 29° and 66°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (29° + 66°) = 85°.
6. Two interior angles of a triangle measure 27° and 72°. Find the third angle.
81°
Angles in a triangle sum to 180°: 180° − (27° + 72°) = 81°.
7. Two interior angles of a triangle measure 68° and 56°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (68° + 56°) = 56°.
8. Two interior angles of a triangle measure 66° and 75°. Find the third angle.
39°
Angles in a triangle sum to 180°: 180° − (66° + 75°) = 39°.
9. Two interior angles of a triangle measure 42° and 25°. Find the third angle.
113°
Angles in a triangle sum to 180°: 180° − (42° + 25°) = 113°.
10. Two interior angles of a triangle measure 40° and 42°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (40° + 42°) = 98°.
11. Two interior angles of a triangle measure 84° and 54°. Find the third angle.
42°
Angles in a triangle sum to 180°: 180° − (84° + 54°) = 42°.
12. Two interior angles of a triangle measure 55° and 40°. Find the third angle.
85°
Angles in a triangle sum to 180°: 180° − (55° + 40°) = 85°.

12 questions · Seed 104737 · Age guidance, not curriculum certification