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 51° and 27°. Find the third angle.
51° · 27° · Missing triangle angles
2. Two interior angles of a triangle measure 42° and 68°. Find the third angle.
42° · 68° · Missing triangle angles
3. Two interior angles of a triangle measure 59° and 66°. Find the third angle.
59° · 66° · Missing triangle angles
4. Two interior angles of a triangle measure 26° and 37°. Find the third angle.
26° · 37° · Missing triangle angles
5. Two interior angles of a triangle measure 25° and 50°. Find the third angle.
25° · 50° · Missing triangle angles
6. Two interior angles of a triangle measure 47° and 72°. Find the third angle.
47° · 72° · Missing triangle angles
7. Two interior angles of a triangle measure 36° and 55°. Find the third angle.
36° · 55° · Missing triangle angles
8. Two interior angles of a triangle measure 32° and 74°. Find the third angle.
32° · 74° · Missing triangle angles
9. Two interior angles of a triangle measure 76° and 35°. Find the third angle.
76° · 35° · Missing triangle angles
10. Two interior angles of a triangle measure 50° and 31°. Find the third angle.
50° · 31° · Missing triangle angles
11. Two interior angles of a triangle measure 57° and 67°. Find the third angle.
57° · 67° · Missing triangle angles
12. Two interior angles of a triangle measure 28° and 58°. Find the third angle.
28° · 58° · Missing triangle angles

12 questions · Seed 64 · 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 51° and 27°. Find the third angle.
102°
Angles in a triangle sum to 180°: 180° − (51° + 27°) = 102°.
2. Two interior angles of a triangle measure 42° and 68°. Find the third angle.
70°
Angles in a triangle sum to 180°: 180° − (42° + 68°) = 70°.
3. Two interior angles of a triangle measure 59° and 66°. Find the third angle.
55°
Angles in a triangle sum to 180°: 180° − (59° + 66°) = 55°.
4. Two interior angles of a triangle measure 26° and 37°. Find the third angle.
117°
Angles in a triangle sum to 180°: 180° − (26° + 37°) = 117°.
5. Two interior angles of a triangle measure 25° and 50°. Find the third angle.
105°
Angles in a triangle sum to 180°: 180° − (25° + 50°) = 105°.
6. Two interior angles of a triangle measure 47° and 72°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (47° + 72°) = 61°.
7. Two interior angles of a triangle measure 36° and 55°. Find the third angle.
89°
Angles in a triangle sum to 180°: 180° − (36° + 55°) = 89°.
8. Two interior angles of a triangle measure 32° and 74°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (32° + 74°) = 74°.
9. Two interior angles of a triangle measure 76° and 35°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (76° + 35°) = 69°.
10. Two interior angles of a triangle measure 50° and 31°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (50° + 31°) = 99°.
11. Two interior angles of a triangle measure 57° and 67°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (57° + 67°) = 56°.
12. Two interior angles of a triangle measure 28° and 58°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (28° + 58°) = 94°.

12 questions · Seed 64 · 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 51° and 27°. Find the third angle.
51° · 27° · Missing triangle angles
2. Two interior angles of a triangle measure 42° and 68°. Find the third angle.
42° · 68° · Missing triangle angles
3. Two interior angles of a triangle measure 59° and 66°. Find the third angle.
59° · 66° · Missing triangle angles
4. Two interior angles of a triangle measure 26° and 37°. Find the third angle.
26° · 37° · Missing triangle angles
5. Two interior angles of a triangle measure 25° and 50°. Find the third angle.
25° · 50° · Missing triangle angles
6. Two interior angles of a triangle measure 47° and 72°. Find the third angle.
47° · 72° · Missing triangle angles
7. Two interior angles of a triangle measure 36° and 55°. Find the third angle.
36° · 55° · Missing triangle angles
8. Two interior angles of a triangle measure 32° and 74°. Find the third angle.
32° · 74° · Missing triangle angles
9. Two interior angles of a triangle measure 76° and 35°. Find the third angle.
76° · 35° · Missing triangle angles
10. Two interior angles of a triangle measure 50° and 31°. Find the third angle.
50° · 31° · Missing triangle angles
11. Two interior angles of a triangle measure 57° and 67°. Find the third angle.
57° · 67° · Missing triangle angles
12. Two interior angles of a triangle measure 28° and 58°. Find the third angle.
28° · 58° · Missing triangle angles

12 questions · Seed 64 · 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 51° and 27°. Find the third angle.
102°
Angles in a triangle sum to 180°: 180° − (51° + 27°) = 102°.
2. Two interior angles of a triangle measure 42° and 68°. Find the third angle.
70°
Angles in a triangle sum to 180°: 180° − (42° + 68°) = 70°.
3. Two interior angles of a triangle measure 59° and 66°. Find the third angle.
55°
Angles in a triangle sum to 180°: 180° − (59° + 66°) = 55°.
4. Two interior angles of a triangle measure 26° and 37°. Find the third angle.
117°
Angles in a triangle sum to 180°: 180° − (26° + 37°) = 117°.
5. Two interior angles of a triangle measure 25° and 50°. Find the third angle.
105°
Angles in a triangle sum to 180°: 180° − (25° + 50°) = 105°.
6. Two interior angles of a triangle measure 47° and 72°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (47° + 72°) = 61°.
7. Two interior angles of a triangle measure 36° and 55°. Find the third angle.
89°
Angles in a triangle sum to 180°: 180° − (36° + 55°) = 89°.
8. Two interior angles of a triangle measure 32° and 74°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (32° + 74°) = 74°.
9. Two interior angles of a triangle measure 76° and 35°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (76° + 35°) = 69°.
10. Two interior angles of a triangle measure 50° and 31°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (50° + 31°) = 99°.
11. Two interior angles of a triangle measure 57° and 67°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (57° + 67°) = 56°.
12. Two interior angles of a triangle measure 28° and 58°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (28° + 58°) = 94°.

12 questions · Seed 64 · Age guidance, not curriculum certification