ReasoningAges 13-15

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 13-15

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 51° and 71°. Find the third angle.
51° · 71° · Missing triangle angles
2. Two interior angles of a triangle measure 63° and 28°. Find the third angle.
63° · 28° · Missing triangle angles
3. Two interior angles of a triangle measure 71° and 46°. Find the third angle.
71° · 46° · Missing triangle angles
4. Two interior angles of a triangle measure 29° and 57°. Find the third angle.
29° · 57° · Missing triangle angles
5. Two interior angles of a triangle measure 66° and 67°. Find the third angle.
66° · 67° · Missing triangle angles
6. Two interior angles of a triangle measure 34° and 25°. Find the third angle.
34° · 25° · Missing triangle angles
7. Two interior angles of a triangle measure 65° and 44°. Find the third angle.
65° · 44° · Missing triangle angles
8. Two interior angles of a triangle measure 81° and 26°. Find the third angle.
81° · 26° · Missing triangle angles
9. Two interior angles of a triangle measure 33° and 49°. Find the third angle.
33° · 49° · Missing triangle angles
10. Two interior angles of a triangle measure 80° and 39°. Find the third angle.
80° · 39° · Missing triangle angles
11. Two interior angles of a triangle measure 74° and 37°. Find the third angle.
74° · 37° · Missing triangle angles
12. Two interior angles of a triangle measure 85° and 25°. Find the third angle.
85° · 25° · Missing triangle angles

12 questions · Seed 56 · Age guidance, not curriculum certification

View the answer key

Worksheet StudioAnswer key · Ages 13-15

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 51° and 71°. Find the third angle.
58°
Angles in a triangle sum to 180°: 180° − (51° + 71°) = 58°.
2. Two interior angles of a triangle measure 63° and 28°. Find the third angle.
89°
Angles in a triangle sum to 180°: 180° − (63° + 28°) = 89°.
3. Two interior angles of a triangle measure 71° and 46°. Find the third angle.
63°
Angles in a triangle sum to 180°: 180° − (71° + 46°) = 63°.
4. Two interior angles of a triangle measure 29° and 57°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (29° + 57°) = 94°.
5. Two interior angles of a triangle measure 66° and 67°. Find the third angle.
47°
Angles in a triangle sum to 180°: 180° − (66° + 67°) = 47°.
6. Two interior angles of a triangle measure 34° and 25°. Find the third angle.
121°
Angles in a triangle sum to 180°: 180° − (34° + 25°) = 121°.
7. Two interior angles of a triangle measure 65° and 44°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (65° + 44°) = 71°.
8. Two interior angles of a triangle measure 81° and 26°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (81° + 26°) = 73°.
9. Two interior angles of a triangle measure 33° and 49°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (33° + 49°) = 98°.
10. Two interior angles of a triangle measure 80° and 39°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (80° + 39°) = 61°.
11. Two interior angles of a triangle measure 74° and 37°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (74° + 37°) = 69°.
12. Two interior angles of a triangle measure 85° and 25°. Find the third angle.
70°
Angles in a triangle sum to 180°: 180° − (85° + 25°) = 70°.

12 questions · Seed 56 · Age guidance, not curriculum certification

Worksheet StudioPractice pack · Ages 13-15

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 51° and 71°. Find the third angle.
51° · 71° · Missing triangle angles
2. Two interior angles of a triangle measure 63° and 28°. Find the third angle.
63° · 28° · Missing triangle angles
3. Two interior angles of a triangle measure 71° and 46°. Find the third angle.
71° · 46° · Missing triangle angles
4. Two interior angles of a triangle measure 29° and 57°. Find the third angle.
29° · 57° · Missing triangle angles
5. Two interior angles of a triangle measure 66° and 67°. Find the third angle.
66° · 67° · Missing triangle angles
6. Two interior angles of a triangle measure 34° and 25°. Find the third angle.
34° · 25° · Missing triangle angles
7. Two interior angles of a triangle measure 65° and 44°. Find the third angle.
65° · 44° · Missing triangle angles
8. Two interior angles of a triangle measure 81° and 26°. Find the third angle.
81° · 26° · Missing triangle angles
9. Two interior angles of a triangle measure 33° and 49°. Find the third angle.
33° · 49° · Missing triangle angles
10. Two interior angles of a triangle measure 80° and 39°. Find the third angle.
80° · 39° · Missing triangle angles
11. Two interior angles of a triangle measure 74° and 37°. Find the third angle.
74° · 37° · Missing triangle angles
12. Two interior angles of a triangle measure 85° and 25°. Find the third angle.
85° · 25° · Missing triangle angles

12 questions · Seed 56 · Age guidance, not curriculum certification

Worksheet StudioAnswer key · Ages 13-15

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 51° and 71°. Find the third angle.
58°
Angles in a triangle sum to 180°: 180° − (51° + 71°) = 58°.
2. Two interior angles of a triangle measure 63° and 28°. Find the third angle.
89°
Angles in a triangle sum to 180°: 180° − (63° + 28°) = 89°.
3. Two interior angles of a triangle measure 71° and 46°. Find the third angle.
63°
Angles in a triangle sum to 180°: 180° − (71° + 46°) = 63°.
4. Two interior angles of a triangle measure 29° and 57°. Find the third angle.
94°
Angles in a triangle sum to 180°: 180° − (29° + 57°) = 94°.
5. Two interior angles of a triangle measure 66° and 67°. Find the third angle.
47°
Angles in a triangle sum to 180°: 180° − (66° + 67°) = 47°.
6. Two interior angles of a triangle measure 34° and 25°. Find the third angle.
121°
Angles in a triangle sum to 180°: 180° − (34° + 25°) = 121°.
7. Two interior angles of a triangle measure 65° and 44°. Find the third angle.
71°
Angles in a triangle sum to 180°: 180° − (65° + 44°) = 71°.
8. Two interior angles of a triangle measure 81° and 26°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (81° + 26°) = 73°.
9. Two interior angles of a triangle measure 33° and 49°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (33° + 49°) = 98°.
10. Two interior angles of a triangle measure 80° and 39°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (80° + 39°) = 61°.
11. Two interior angles of a triangle measure 74° and 37°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (74° + 37°) = 69°.
12. Two interior angles of a triangle measure 85° and 25°. Find the third angle.
70°
Angles in a triangle sum to 180°: 180° − (85° + 25°) = 70°.

12 questions · Seed 56 · Age guidance, not curriculum certification