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 55° and 26°. Find the third angle.
55° · 26° · Missing triangle angles
2. Two interior angles of a triangle measure 40° and 48°. Find the third angle.
40° · 48° · Missing triangle angles
3. Two interior angles of a triangle measure 69° and 25°. Find the third angle.
69° · 25° · Missing triangle angles
4. Two interior angles of a triangle measure 48° and 45°. Find the third angle.
48° · 45° · Missing triangle angles
5. Two interior angles of a triangle measure 26° and 37°. Find the third angle.
26° · 37° · Missing triangle angles
6. Two interior angles of a triangle measure 73° and 34°. Find the third angle.
73° · 34° · Missing triangle angles
7. Two interior angles of a triangle measure 61° and 32°. Find the third angle.
61° · 32° · Missing triangle angles
8. Two interior angles of a triangle measure 27° and 73°. Find the third angle.
27° · 73° · Missing triangle angles
9. Two interior angles of a triangle measure 61° and 31°. Find the third angle.
61° · 31° · Missing triangle angles
10. Two interior angles of a triangle measure 80° and 34°. Find the third angle.
80° · 34° · Missing triangle angles
11. Two interior angles of a triangle measure 45° and 37°. Find the third angle.
45° · 37° · Missing triangle angles
12. Two interior angles of a triangle measure 50° and 35°. Find the third angle.
50° · 35° · Missing triangle angles

12 questions · Seed 104729 · 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 55° and 26°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (55° + 26°) = 99°.
2. Two interior angles of a triangle measure 40° and 48°. Find the third angle.
92°
Angles in a triangle sum to 180°: 180° − (40° + 48°) = 92°.
3. Two interior angles of a triangle measure 69° and 25°. Find the third angle.
86°
Angles in a triangle sum to 180°: 180° − (69° + 25°) = 86°.
4. Two interior angles of a triangle measure 48° and 45°. Find the third angle.
87°
Angles in a triangle sum to 180°: 180° − (48° + 45°) = 87°.
5. 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°.
6. Two interior angles of a triangle measure 73° and 34°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (73° + 34°) = 73°.
7. Two interior angles of a triangle measure 61° and 32°. Find the third angle.
87°
Angles in a triangle sum to 180°: 180° − (61° + 32°) = 87°.
8. Two interior angles of a triangle measure 27° and 73°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (27° + 73°) = 80°.
9. Two interior angles of a triangle measure 61° and 31°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (61° + 31°) = 88°.
10. Two interior angles of a triangle measure 80° and 34°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (80° + 34°) = 66°.
11. Two interior angles of a triangle measure 45° and 37°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (45° + 37°) = 98°.
12. Two interior angles of a triangle measure 50° and 35°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (50° + 35°) = 95°.

12 questions · Seed 104729 · 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 55° and 26°. Find the third angle.
55° · 26° · Missing triangle angles
2. Two interior angles of a triangle measure 40° and 48°. Find the third angle.
40° · 48° · Missing triangle angles
3. Two interior angles of a triangle measure 69° and 25°. Find the third angle.
69° · 25° · Missing triangle angles
4. Two interior angles of a triangle measure 48° and 45°. Find the third angle.
48° · 45° · Missing triangle angles
5. Two interior angles of a triangle measure 26° and 37°. Find the third angle.
26° · 37° · Missing triangle angles
6. Two interior angles of a triangle measure 73° and 34°. Find the third angle.
73° · 34° · Missing triangle angles
7. Two interior angles of a triangle measure 61° and 32°. Find the third angle.
61° · 32° · Missing triangle angles
8. Two interior angles of a triangle measure 27° and 73°. Find the third angle.
27° · 73° · Missing triangle angles
9. Two interior angles of a triangle measure 61° and 31°. Find the third angle.
61° · 31° · Missing triangle angles
10. Two interior angles of a triangle measure 80° and 34°. Find the third angle.
80° · 34° · Missing triangle angles
11. Two interior angles of a triangle measure 45° and 37°. Find the third angle.
45° · 37° · Missing triangle angles
12. Two interior angles of a triangle measure 50° and 35°. Find the third angle.
50° · 35° · Missing triangle angles

12 questions · Seed 104729 · 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 55° and 26°. Find the third angle.
99°
Angles in a triangle sum to 180°: 180° − (55° + 26°) = 99°.
2. Two interior angles of a triangle measure 40° and 48°. Find the third angle.
92°
Angles in a triangle sum to 180°: 180° − (40° + 48°) = 92°.
3. Two interior angles of a triangle measure 69° and 25°. Find the third angle.
86°
Angles in a triangle sum to 180°: 180° − (69° + 25°) = 86°.
4. Two interior angles of a triangle measure 48° and 45°. Find the third angle.
87°
Angles in a triangle sum to 180°: 180° − (48° + 45°) = 87°.
5. 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°.
6. Two interior angles of a triangle measure 73° and 34°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (73° + 34°) = 73°.
7. Two interior angles of a triangle measure 61° and 32°. Find the third angle.
87°
Angles in a triangle sum to 180°: 180° − (61° + 32°) = 87°.
8. Two interior angles of a triangle measure 27° and 73°. Find the third angle.
80°
Angles in a triangle sum to 180°: 180° − (27° + 73°) = 80°.
9. Two interior angles of a triangle measure 61° and 31°. Find the third angle.
88°
Angles in a triangle sum to 180°: 180° − (61° + 31°) = 88°.
10. Two interior angles of a triangle measure 80° and 34°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (80° + 34°) = 66°.
11. Two interior angles of a triangle measure 45° and 37°. Find the third angle.
98°
Angles in a triangle sum to 180°: 180° − (45° + 37°) = 98°.
12. Two interior angles of a triangle measure 50° and 35°. Find the third angle.
95°
Angles in a triangle sum to 180°: 180° − (50° + 35°) = 95°.

12 questions · Seed 104729 · Age guidance, not curriculum certification