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 84° and 29°. Find the third angle.
84° · 29° · Missing triangle angles
2. Two interior angles of a triangle measure 42° and 33°. Find the third angle.
42° · 33° · Missing triangle angles
3. Two interior angles of a triangle measure 65° and 31°. Find the third angle.
65° · 31° · Missing triangle angles
4. Two interior angles of a triangle measure 44° and 75°. Find the third angle.
44° · 75° · Missing triangle angles
5. Two interior angles of a triangle measure 64° and 55°. Find the third angle.
64° · 55° · Missing triangle angles
6. Two interior angles of a triangle measure 35° and 37°. Find the third angle.
35° · 37° · Missing triangle angles
7. Two interior angles of a triangle measure 42° and 39°. Find the third angle.
42° · 39° · Missing triangle angles
8. Two interior angles of a triangle measure 32° and 56°. Find the third angle.
32° · 56° · Missing triangle angles
9. Two interior angles of a triangle measure 36° and 64°. Find the third angle.
36° · 64° · Missing triangle angles
10. Two interior angles of a triangle measure 63° and 36°. Find the third angle.
63° · 36° · Missing triangle angles
11. Two interior angles of a triangle measure 70° and 40°. Find the third angle.
70° · 40° · Missing triangle angles
12. Two interior angles of a triangle measure 66° and 72°. Find the third angle.
66° · 72° · Missing triangle angles

12 questions · Seed 66 · 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 84° and 29°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (84° + 29°) = 67°.
2. Two interior angles of a triangle measure 42° and 33°. Find the third angle.
105°
Angles in a triangle sum to 180°: 180° − (42° + 33°) = 105°.
3. Two interior angles of a triangle measure 65° and 31°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (65° + 31°) = 84°.
4. Two interior angles of a triangle measure 44° and 75°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (44° + 75°) = 61°.
5. Two interior angles of a triangle measure 64° and 55°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (64° + 55°) = 61°.
6. Two interior angles of a triangle measure 35° and 37°. Find the third angle.
108°
Angles in a triangle sum to 180°: 180° − (35° + 37°) = 108°.
7. Two interior angles of a triangle measure 42° and 39°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (42° + 39°) = 99°.
8. Two interior angles of a triangle measure 32° and 56°. Find the third angle.
92°
Angles in a triangle sum to 180°: 180° − (32° + 56°) = 92°.
9. Two interior angles of a triangle measure 36° and 64°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (36° + 64°) = 80°.
10. Two interior angles of a triangle measure 63° and 36°. Find the third angle.
81°
Angles in a triangle sum to 180°: 180° − (63° + 36°) = 81°.
11. Two interior angles of a triangle measure 70° and 40°. Find the third angle.
70°
Angles in a triangle sum to 180°: 180° − (70° + 40°) = 70°.
12. Two interior angles of a triangle measure 66° and 72°. Find the third angle.
42°
Angles in a triangle sum to 180°: 180° − (66° + 72°) = 42°.

12 questions · Seed 66 · 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 84° and 29°. Find the third angle.
84° · 29° · Missing triangle angles
2. Two interior angles of a triangle measure 42° and 33°. Find the third angle.
42° · 33° · Missing triangle angles
3. Two interior angles of a triangle measure 65° and 31°. Find the third angle.
65° · 31° · Missing triangle angles
4. Two interior angles of a triangle measure 44° and 75°. Find the third angle.
44° · 75° · Missing triangle angles
5. Two interior angles of a triangle measure 64° and 55°. Find the third angle.
64° · 55° · Missing triangle angles
6. Two interior angles of a triangle measure 35° and 37°. Find the third angle.
35° · 37° · Missing triangle angles
7. Two interior angles of a triangle measure 42° and 39°. Find the third angle.
42° · 39° · Missing triangle angles
8. Two interior angles of a triangle measure 32° and 56°. Find the third angle.
32° · 56° · Missing triangle angles
9. Two interior angles of a triangle measure 36° and 64°. Find the third angle.
36° · 64° · Missing triangle angles
10. Two interior angles of a triangle measure 63° and 36°. Find the third angle.
63° · 36° · Missing triangle angles
11. Two interior angles of a triangle measure 70° and 40°. Find the third angle.
70° · 40° · Missing triangle angles
12. Two interior angles of a triangle measure 66° and 72°. Find the third angle.
66° · 72° · Missing triangle angles

12 questions · Seed 66 · 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 84° and 29°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (84° + 29°) = 67°.
2. Two interior angles of a triangle measure 42° and 33°. Find the third angle.
105°
Angles in a triangle sum to 180°: 180° − (42° + 33°) = 105°.
3. Two interior angles of a triangle measure 65° and 31°. Find the third angle.
84°
Angles in a triangle sum to 180°: 180° − (65° + 31°) = 84°.
4. Two interior angles of a triangle measure 44° and 75°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (44° + 75°) = 61°.
5. Two interior angles of a triangle measure 64° and 55°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (64° + 55°) = 61°.
6. Two interior angles of a triangle measure 35° and 37°. Find the third angle.
108°
Angles in a triangle sum to 180°: 180° − (35° + 37°) = 108°.
7. Two interior angles of a triangle measure 42° and 39°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (42° + 39°) = 99°.
8. Two interior angles of a triangle measure 32° and 56°. Find the third angle.
92°
Angles in a triangle sum to 180°: 180° − (32° + 56°) = 92°.
9. Two interior angles of a triangle measure 36° and 64°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (36° + 64°) = 80°.
10. Two interior angles of a triangle measure 63° and 36°. Find the third angle.
81°
Angles in a triangle sum to 180°: 180° − (63° + 36°) = 81°.
11. Two interior angles of a triangle measure 70° and 40°. Find the third angle.
70°
Angles in a triangle sum to 180°: 180° − (70° + 40°) = 70°.
12. Two interior angles of a triangle measure 66° and 72°. Find the third angle.
42°
Angles in a triangle sum to 180°: 180° − (66° + 72°) = 42°.

12 questions · Seed 66 · Age guidance, not curriculum certification