Angles in a triangle.
Work out the sizes of the unknown angles.

🙂 = Correct, ☹️ = Incorrect

📌 Rules

• Interior angles in any triangle add up to 180°: To find a missing angle, add the two known angles together and subtract their total from 180° (e.g. 180° – (60° + 70°) = 50°).

• Right-angled triangles contain a 90° angle: Look for the square symbol (∟). Subtract the known acute angle from 90° to find the remaining angle (e.g. 90° – 35° = 55°).

• Isosceles triangles have two equal angles: The angles opposite the two equal sides are equal in size. If the top angle is 40°, subtract from 180° and divide by 2 to find each base angle (e.g. (180° – 40°) ÷ 2 = 70°).

• Equilateral triangles have three equal angles: All three interior angles are always equal to 60° (180° ÷ 3 = 60°).

💡 Tips

• Identify angle types to double-check: Acute angles are under 90°, obtuse angles are between 90° and 180°, and right angles are exactly 90°.

• Look for tick marks on sides: Matching tick marks on triangle sides show they are equal in length, indicating an isosceles or equilateral triangle.

• Quick typing on our platform: Typing your answer is designed to be super quick and simple! Just type plain numbers like 50, 55, or 70 without adding the degree symbol (°), letters, or spaces.

⚠️ Warnings

• Do not use a protractor unless instructed: Test diagrams are rarely drawn to scale! Always use calculation rather than measuring.

• Don’t forget the square marker means 90°: If a right-angled triangle has a square symbol (∟), remember that angle counts as 90° even if the number is not written out.