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.

New variation Try interactive challenge Browse templates

Age band:Ages 10-12 Ages 13-15

Choose “Save as PDF” to keep a copy. Use your browser’s paper setting for A4 or Letter.

Worksheet StudioPractice pack · Ages 10-12

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 71° and 50°. Find the third angle.
71° · 50° · Missing triangle angles
2. Two interior angles of a triangle measure 69° and 40°. Find the third angle.
69° · 40° · Missing triangle angles
3. Two interior angles of a triangle measure 73° and 35°. Find the third angle.
73° · 35° · Missing triangle angles
4. Two interior angles of a triangle measure 50° and 59°. Find the third angle.
50° · 59° · Missing triangle angles
5. Two interior angles of a triangle measure 37° and 60°. Find the third angle.
37° · 60° · Missing triangle angles
6. Two interior angles of a triangle measure 43° and 57°. Find the third angle.
43° · 57° · Missing triangle angles
7. Two interior angles of a triangle measure 39° and 43°. Find the third angle.
39° · 43° · Missing triangle angles
8. Two interior angles of a triangle measure 45° and 32°. Find the third angle.
45° · 32° · Missing triangle angles
9. Two interior angles of a triangle measure 85° and 66°. Find the third angle.
85° · 66° · Missing triangle angles
10. Two interior angles of a triangle measure 71° and 27°. Find the third angle.
71° · 27° · Missing triangle angles
11. Two interior angles of a triangle measure 36° and 64°. Find the third angle.
36° · 64° · Missing triangle angles
12. Two interior angles of a triangle measure 59° and 33°. Find the third angle.
59° · 33° · Missing triangle angles

12 questions · Seed 62 · 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 71° and 50°. Find the third angle.
59°
Angles in a triangle sum to 180°: 180° − (71° + 50°) = 59°.
2. Two interior angles of a triangle measure 69° and 40°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (69° + 40°) = 71°.
3. Two interior angles of a triangle measure 73° and 35°. Find the third angle.
72°
Angles in a triangle sum to 180°: 180° − (73° + 35°) = 72°.
4. Two interior angles of a triangle measure 50° and 59°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (50° + 59°) = 71°.
5. Two interior angles of a triangle measure 37° and 60°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (37° + 60°) = 83°.
6. Two interior angles of a triangle measure 43° and 57°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (43° + 57°) = 80°.
7. Two interior angles of a triangle measure 39° and 43°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (39° + 43°) = 98°.
8. Two interior angles of a triangle measure 45° and 32°. Find the third angle.
103°
Angles in a triangle sum to 180°: 180° − (45° + 32°) = 103°.
9. Two interior angles of a triangle measure 85° and 66°. Find the third angle.
29°
Angles in a triangle sum to 180°: 180° − (85° + 66°) = 29°.
10. Two interior angles of a triangle measure 71° and 27°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (71° + 27°) = 82°.
11. 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°.
12. Two interior angles of a triangle measure 59° and 33°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (59° + 33°) = 88°.

12 questions · Seed 62 · 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 71° and 50°. Find the third angle.
71° · 50° · Missing triangle angles
2. Two interior angles of a triangle measure 69° and 40°. Find the third angle.
69° · 40° · Missing triangle angles
3. Two interior angles of a triangle measure 73° and 35°. Find the third angle.
73° · 35° · Missing triangle angles
4. Two interior angles of a triangle measure 50° and 59°. Find the third angle.
50° · 59° · Missing triangle angles
5. Two interior angles of a triangle measure 37° and 60°. Find the third angle.
37° · 60° · Missing triangle angles
6. Two interior angles of a triangle measure 43° and 57°. Find the third angle.
43° · 57° · Missing triangle angles
7. Two interior angles of a triangle measure 39° and 43°. Find the third angle.
39° · 43° · Missing triangle angles
8. Two interior angles of a triangle measure 45° and 32°. Find the third angle.
45° · 32° · Missing triangle angles
9. Two interior angles of a triangle measure 85° and 66°. Find the third angle.
85° · 66° · Missing triangle angles
10. Two interior angles of a triangle measure 71° and 27°. Find the third angle.
71° · 27° · Missing triangle angles
11. Two interior angles of a triangle measure 36° and 64°. Find the third angle.
36° · 64° · Missing triangle angles
12. Two interior angles of a triangle measure 59° and 33°. Find the third angle.
59° · 33° · Missing triangle angles

12 questions · Seed 62 · 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 71° and 50°. Find the third angle.
59°
Angles in a triangle sum to 180°: 180° − (71° + 50°) = 59°.
2. Two interior angles of a triangle measure 69° and 40°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (69° + 40°) = 71°.
3. Two interior angles of a triangle measure 73° and 35°. Find the third angle.
72°
Angles in a triangle sum to 180°: 180° − (73° + 35°) = 72°.
4. Two interior angles of a triangle measure 50° and 59°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (50° + 59°) = 71°.
5. Two interior angles of a triangle measure 37° and 60°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (37° + 60°) = 83°.
6. Two interior angles of a triangle measure 43° and 57°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (43° + 57°) = 80°.
7. Two interior angles of a triangle measure 39° and 43°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (39° + 43°) = 98°.
8. Two interior angles of a triangle measure 45° and 32°. Find the third angle.
103°
Angles in a triangle sum to 180°: 180° − (45° + 32°) = 103°.
9. Two interior angles of a triangle measure 85° and 66°. Find the third angle.
29°
Angles in a triangle sum to 180°: 180° − (85° + 66°) = 29°.
10. Two interior angles of a triangle measure 71° and 27°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (71° + 27°) = 82°.
11. 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°.
12. Two interior angles of a triangle measure 59° and 33°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (59° + 33°) = 88°.

12 questions · Seed 62 · Age guidance, not curriculum certification