Area of a Triangle Worksheets - Math Monks - Free Printable

Let’s solve each triangle one by one. Remember: The sum of all three angles in any triangle is always 180°. We’ll use that to find the missing angle in each case.

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Problem 1:
Given angles: 59° and 67°
Missing angle = 180° - (59° + 67°) = 180° - 126° = 54°

Problem 2:
Given angles: 45° and 90°
Missing angle = 180° - (45° + 90°) = 180° - 135° = 45°

Problem 3:
Given angles: 20° and 129°
Missing angle = 180° - (20° + 129°) = 180° - 149° = 31°

Problem 4:
This looks like a triangle with two base angles given: 30° and 25°, and we’re asked for the top angle (the one at the intersection).
Missing angle = 180° - (30° + 25°) = 180° - 55° = 125°

*(Note: The lines cross, but the triangle formed has those two bottom angles — so same rule applies.)*

Problem 5:
Given angles: 65° and 33°
Missing angle = 180° - (65° + 33°) = 180° - 98° = 82°

Problem 6:
Given angles: 47° and 90°
Missing angle = 180° - (47° + 90°) = 180° - 137° = 43°

Problem 7:
Given angles: 77° and 29°
Missing angle = 180° - (77° + 29°) = 180° - 106° = 74°

Problem 8:
Given angles: 66° and 35°
Missing angle = 180° - (66° + 35°) = 180° - 101° = 79°

Problem 9:
Given angles: 70° and 29°
Wait — look carefully! The 70° is outside the triangle? No — actually, it’s labeled on the inside vertex. So yes, both are interior angles.
Missing angle = 180° - (70° + 29°) = 180° - 99° = 81°

*(Double-checking: Yes, all three angles are inside the triangle.)*

Problem 10:
Given angles: 40° and 50°
But wait — there’s an exterior angle shown. Actually, the question asks for the unknown angle *in the triangle*. The third angle inside the triangle is what we need.
So: Missing interior angle = 180° - (40° + 50°) = 180° - 90° = 90°

*(The line extending out is just showing the exterior angle — but we only care about the triangle’s internal angles.)*

Problem 11:
Given angles: 20° and 50°
Missing angle = 180° - (20° + 50°) = 180° - 70° = 110°

Problem 12:
Given angles: 30° and 75°
Missing angle = 180° - (30° + 75°) = 180° - 105° = 75°

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✔ All calculations double-checked using 180° total per triangle.

Final Answer:
1. 54°
2. 45°
3. 31°
4. 125°
5. 82°
6. 43°
7. 74°
8. 79°
9. 81°
10. 90°
11. 110°
12. 75°