Geometry

Adjacent Angles

Why they always add up to 180°

  • Primary school
  • about 3 min
  • 1 of 9 in the family
Not started
Vαβshared armthe other two arms are opposite rays
0°straight angle = 180°180°
α 55°
β 125°
Swing the shared armα = 55°
α55°
β125°
α + β180°

Try any position of the arm – the sum never changes.

Adjacent angles share one arm while their other two arms are opposite rays. Together they fill exactly a straight angle: α + β = 180°.