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 58° and 47°. Find the third angle.
58° · 47° · Missing triangle angles
2. Two interior angles of a triangle measure 26° and 46°. Find the third angle.
26° · 46° · Missing triangle angles
3. Two interior angles of a triangle measure 79° and 25°. Find the third angle.
79° · 25° · Missing triangle angles
4. Two interior angles of a triangle measure 26° and 47°. Find the third angle.
26° · 47° · Missing triangle angles
5. Two interior angles of a triangle measure 45° and 52°. Find the third angle.
45° · 52° · Missing triangle angles
6. Two interior angles of a triangle measure 56° and 55°. Find the third angle.
56° · 55° · Missing triangle angles
7. Two interior angles of a triangle measure 68° and 56°. Find the third angle.
68° · 56° · Missing triangle angles
8. Two interior angles of a triangle measure 42° and 29°. Find the third angle.
42° · 29° · Missing triangle angles
9. Two interior angles of a triangle measure 54° and 52°. Find the third angle.
54° · 52° · Missing triangle angles
10. Two interior angles of a triangle measure 51° and 63°. Find the third angle.
51° · 63° · Missing triangle angles
11. Two interior angles of a triangle measure 62° and 27°. Find the third angle.
62° · 27° · Missing triangle angles
12. Two interior angles of a triangle measure 65° and 59°. Find the third angle.
65° · 59° · Missing triangle angles

12 questions · Seed 69 · 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 58° and 47°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (58° + 47°) = 75°.
2. Two interior angles of a triangle measure 26° and 46°. Find the third angle.
108°
Angles in a triangle sum to 180°: 180° − (26° + 46°) = 108°.
3. Two interior angles of a triangle measure 79° and 25°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (79° + 25°) = 76°.
4. Two interior angles of a triangle measure 26° and 47°. Find the third angle.
107°
Angles in a triangle sum to 180°: 180° − (26° + 47°) = 107°.
5. Two interior angles of a triangle measure 45° and 52°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (45° + 52°) = 83°.
6. Two interior angles of a triangle measure 56° and 55°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (56° + 55°) = 69°.
7. Two interior angles of a triangle measure 68° and 56°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (68° + 56°) = 56°.
8. Two interior angles of a triangle measure 42° and 29°. Find the third angle.
109°
Angles in a triangle sum to 180°: 180° − (42° + 29°) = 109°.
9. Two interior angles of a triangle measure 54° and 52°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (54° + 52°) = 74°.
10. Two interior angles of a triangle measure 51° and 63°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (51° + 63°) = 66°.
11. Two interior angles of a triangle measure 62° and 27°. Find the third angle.
91°
Angles in a triangle sum to 180°: 180° − (62° + 27°) = 91°.
12. Two interior angles of a triangle measure 65° and 59°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (65° + 59°) = 56°.

12 questions · Seed 69 · 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 58° and 47°. Find the third angle.
58° · 47° · Missing triangle angles
2. Two interior angles of a triangle measure 26° and 46°. Find the third angle.
26° · 46° · Missing triangle angles
3. Two interior angles of a triangle measure 79° and 25°. Find the third angle.
79° · 25° · Missing triangle angles
4. Two interior angles of a triangle measure 26° and 47°. Find the third angle.
26° · 47° · Missing triangle angles
5. Two interior angles of a triangle measure 45° and 52°. Find the third angle.
45° · 52° · Missing triangle angles
6. Two interior angles of a triangle measure 56° and 55°. Find the third angle.
56° · 55° · Missing triangle angles
7. Two interior angles of a triangle measure 68° and 56°. Find the third angle.
68° · 56° · Missing triangle angles
8. Two interior angles of a triangle measure 42° and 29°. Find the third angle.
42° · 29° · Missing triangle angles
9. Two interior angles of a triangle measure 54° and 52°. Find the third angle.
54° · 52° · Missing triangle angles
10. Two interior angles of a triangle measure 51° and 63°. Find the third angle.
51° · 63° · Missing triangle angles
11. Two interior angles of a triangle measure 62° and 27°. Find the third angle.
62° · 27° · Missing triangle angles
12. Two interior angles of a triangle measure 65° and 59°. Find the third angle.
65° · 59° · Missing triangle angles

12 questions · Seed 69 · 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 58° and 47°. Find the third angle.
75°
Angles in a triangle sum to 180°: 180° − (58° + 47°) = 75°.
2. Two interior angles of a triangle measure 26° and 46°. Find the third angle.
108°
Angles in a triangle sum to 180°: 180° − (26° + 46°) = 108°.
3. Two interior angles of a triangle measure 79° and 25°. Find the third angle.
76°
Angles in a triangle sum to 180°: 180° − (79° + 25°) = 76°.
4. Two interior angles of a triangle measure 26° and 47°. Find the third angle.
107°
Angles in a triangle sum to 180°: 180° − (26° + 47°) = 107°.
5. Two interior angles of a triangle measure 45° and 52°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (45° + 52°) = 83°.
6. Two interior angles of a triangle measure 56° and 55°. Find the third angle.
69°
Angles in a triangle sum to 180°: 180° − (56° + 55°) = 69°.
7. Two interior angles of a triangle measure 68° and 56°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (68° + 56°) = 56°.
8. Two interior angles of a triangle measure 42° and 29°. Find the third angle.
109°
Angles in a triangle sum to 180°: 180° − (42° + 29°) = 109°.
9. Two interior angles of a triangle measure 54° and 52°. Find the third angle.
74°
Angles in a triangle sum to 180°: 180° − (54° + 52°) = 74°.
10. Two interior angles of a triangle measure 51° and 63°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (51° + 63°) = 66°.
11. Two interior angles of a triangle measure 62° and 27°. Find the third angle.
91°
Angles in a triangle sum to 180°: 180° − (62° + 27°) = 91°.
12. Two interior angles of a triangle measure 65° and 59°. Find the third angle.
56°
Angles in a triangle sum to 180°: 180° − (65° + 59°) = 56°.

12 questions · Seed 69 · Age guidance, not curriculum certification