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 49° and 33°. Find the third angle.
49° · 33° · Missing triangle angles
2. Two interior angles of a triangle measure 39° and 75°. Find the third angle.
39° · 75° · Missing triangle angles
3. Two interior angles of a triangle measure 77° and 64°. Find the third angle.
77° · 64° · Missing triangle angles
4. Two interior angles of a triangle measure 47° and 57°. Find the third angle.
47° · 57° · Missing triangle angles
5. Two interior angles of a triangle measure 42° and 63°. Find the third angle.
42° · 63° · Missing triangle angles
6. Two interior angles of a triangle measure 55° and 36°. Find the third angle.
55° · 36° · Missing triangle angles
7. Two interior angles of a triangle measure 82° and 25°. Find the third angle.
82° · 25° · Missing triangle angles
8. Two interior angles of a triangle measure 28° and 64°. Find the third angle.
28° · 64° · Missing triangle angles
9. Two interior angles of a triangle measure 78° and 53°. Find the third angle.
78° · 53° · Missing triangle angles
10. Two interior angles of a triangle measure 43° and 51°. Find the third angle.
43° · 51° · Missing triangle angles
11. Two interior angles of a triangle measure 32° and 32°. Find the third angle.
32° · 32° · Missing triangle angles
12. Two interior angles of a triangle measure 35° and 49°. Find the third angle.
35° · 49° · Missing triangle angles

12 questions · Seed 63 · 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 49° and 33°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (49° + 33°) = 98°.
2. Two interior angles of a triangle measure 39° and 75°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (39° + 75°) = 66°.
3. Two interior angles of a triangle measure 77° and 64°. Find the third angle.
39°
Angles in a triangle sum to 180°: 180° − (77° + 64°) = 39°.
4. Two interior angles of a triangle measure 47° and 57°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (47° + 57°) = 76°.
5. Two interior angles of a triangle measure 42° and 63°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (42° + 63°) = 75°.
6. Two interior angles of a triangle measure 55° and 36°. Find the third angle.
89°
Angles in a triangle sum to 180°: 180° − (55° + 36°) = 89°.
7. Two interior angles of a triangle measure 82° and 25°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (82° + 25°) = 73°.
8. Two interior angles of a triangle measure 28° and 64°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (28° + 64°) = 88°.
9. Two interior angles of a triangle measure 78° and 53°. Find the third angle.
49°
Angles in a triangle sum to 180°: 180° − (78° + 53°) = 49°.
10. Two interior angles of a triangle measure 43° and 51°. Find the third angle.
86°
Angles in a triangle sum to 180°: 180° − (43° + 51°) = 86°.
11. Two interior angles of a triangle measure 32° and 32°. Find the third angle.
116°
Angles in a triangle sum to 180°: 180° − (32° + 32°) = 116°.
12. Two interior angles of a triangle measure 35° and 49°. Find the third angle.
96°
Angles in a triangle sum to 180°: 180° − (35° + 49°) = 96°.

12 questions · Seed 63 · 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 49° and 33°. Find the third angle.
49° · 33° · Missing triangle angles
2. Two interior angles of a triangle measure 39° and 75°. Find the third angle.
39° · 75° · Missing triangle angles
3. Two interior angles of a triangle measure 77° and 64°. Find the third angle.
77° · 64° · Missing triangle angles
4. Two interior angles of a triangle measure 47° and 57°. Find the third angle.
47° · 57° · Missing triangle angles
5. Two interior angles of a triangle measure 42° and 63°. Find the third angle.
42° · 63° · Missing triangle angles
6. Two interior angles of a triangle measure 55° and 36°. Find the third angle.
55° · 36° · Missing triangle angles
7. Two interior angles of a triangle measure 82° and 25°. Find the third angle.
82° · 25° · Missing triangle angles
8. Two interior angles of a triangle measure 28° and 64°. Find the third angle.
28° · 64° · Missing triangle angles
9. Two interior angles of a triangle measure 78° and 53°. Find the third angle.
78° · 53° · Missing triangle angles
10. Two interior angles of a triangle measure 43° and 51°. Find the third angle.
43° · 51° · Missing triangle angles
11. Two interior angles of a triangle measure 32° and 32°. Find the third angle.
32° · 32° · Missing triangle angles
12. Two interior angles of a triangle measure 35° and 49°. Find the third angle.
35° · 49° · Missing triangle angles

12 questions · Seed 63 · 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 49° and 33°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (49° + 33°) = 98°.
2. Two interior angles of a triangle measure 39° and 75°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (39° + 75°) = 66°.
3. Two interior angles of a triangle measure 77° and 64°. Find the third angle.
39°
Angles in a triangle sum to 180°: 180° − (77° + 64°) = 39°.
4. Two interior angles of a triangle measure 47° and 57°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (47° + 57°) = 76°.
5. Two interior angles of a triangle measure 42° and 63°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (42° + 63°) = 75°.
6. Two interior angles of a triangle measure 55° and 36°. Find the third angle.
89°
Angles in a triangle sum to 180°: 180° − (55° + 36°) = 89°.
7. Two interior angles of a triangle measure 82° and 25°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (82° + 25°) = 73°.
8. Two interior angles of a triangle measure 28° and 64°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (28° + 64°) = 88°.
9. Two interior angles of a triangle measure 78° and 53°. Find the third angle.
49°
Angles in a triangle sum to 180°: 180° − (78° + 53°) = 49°.
10. Two interior angles of a triangle measure 43° and 51°. Find the third angle.
86°
Angles in a triangle sum to 180°: 180° − (43° + 51°) = 86°.
11. Two interior angles of a triangle measure 32° and 32°. Find the third angle.
116°
Angles in a triangle sum to 180°: 180° − (32° + 32°) = 116°.
12. Two interior angles of a triangle measure 35° and 49°. Find the third angle.
96°
Angles in a triangle sum to 180°: 180° − (35° + 49°) = 96°.

12 questions · Seed 63 · Age guidance, not curriculum certification