AI Tutor
Open a folding pocket knife halfway, and the blade stands straight up at a right angle. Push it open all the way until it is completely flat, and you have formed a straight angle.
Over 3,000 years ago, Babylonian astronomers tracked the sun across the sky and divided a full circle into 360360 equal degrees. A flat line represents half of that full rotation, which means every straight line measures exactly 180∘180^\circ.
What happens if a ray shoots out from that straight line and splits it into two separate angles?

Linear Pairs and Supplementary Angles

When two adjacent angles share a vertex and a side while sitting on a straight line, they form a linear pair.
Because they make up a straight line, their measures always add up to 180∘180^\circ. Any two angles that add to 180∘180^\circ are called supplementary angles, whether they touch each other or sit completely apart.
Knowing that a straight line is always 180∘180^\circ turns finding a missing angle into simple subtraction.

Calculating Unknown Angles

If you know one angle in a linear pair, subtract it from 180∘180^\circ to find the missing partner: Angle A+Angle B=180∘\text{Angle } A + \text{Angle } B = 180^\circ.
Subtracting 124∘124^\circ from 180∘180^\circ gives x=56∘x = 56^\circ. We can double-check our work instantly: 124∘+56∘=180∘124^\circ + 56^\circ = 180^\circ.