Learn the 3-Step Method
1Know the total: 180°.
True for EVERY triangle — right-angled, isosceles, scalene, all of them.
2Add the known angles.
65° and 40° known: 65 + 40 = 105.
3Subtract from 180.
180 − 105 = 75. The missing angle is 75°.
Worked Example
An isosceles triangle has a top angle of 40°. Find the two base angles.
Watch Out! Common Mistakes
Wrong! 360° is a quadrilateral (or a full turn). A triangle’s angles total 180°.
Look for the square! The little square symbol means 90° — include it: 180 − 90 − 35 = 55°.
Wrong order! Subtract the odd angle first, then halve: (180 − 40) ÷ 2 = 70°.
⬇ Cheat Sheet
- Triangle angles always total 180°.
- Missing angle = 180 − the known two.
- Isosceles: 180 − odd angle, then ÷ 2.
- The little square means 90° — include it.
📖 Continue Your Journey
Quick summary
- The three angles of every triangle total 180°.
- Missing angle = 180° minus the two known angles.
- Isosceles: subtract the odd angle from 180°, then halve for the equal pair.
- A small square in a corner means 90° — count it as a known angle.
Frequently asked questions
Do all triangles' angles add up to 180°?
Yes — equilateral, isosceles, scalene and right-angled alike. It's one of the most reliable facts in geometry.
How do you find missing angles in an isosceles triangle?
Subtract the known odd angle from 180°, then divide by 2 for the two equal base angles: top 40° → (180−40)÷2 = 70° each.
What if the triangle has a right angle?
Count the 90° as one of the known angles: with 35° also known, the third angle is 180 − 90 − 35 = 55°.
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