A straight angle is exactly 180° — the angle along a straight line. Picture two right angles placed back to back, or half of a full turn.
Drag the ray
180°
straight angle
Where 180° shows up
- Angles on a straight line add up to 180° — so if one is 130°, the other must be 50°.
- The angles inside any triangle add to 180° (tear the corners off a paper triangle and they line up flat).
- A half turn — face north, swing 180°, now you face south.
- Two angles that make a straight line are supplementary.
Your turn
Three angles sit along a straight line: 65°, x, and 40°. Find x.
Drag the ray above to 180° — the two arms point in exactly opposite directions and the 'angle' looks like one straight line.