How do you know a straight line forms an angle of 180°?

Question

The question may seem simple. Could we have a line forming an angle of 150° (for example). Thanks.

Answer 1

If two rays form an angle other than $180^\circ$, then it is not a straight line. A straight line, by definition, is $180^\circ$.

A rotation of $150^{\circ}$ is only a fraction of straight angle rotation viz.,

$$\frac {150^{\circ}}{180^{\circ}}=\frac {5}{6}.$$

Answer 2

A degree (in full, a degree of arc, arc degree, or arcdegree), is a measurement of a plane angle, defined so that a full rotation is $360$ degrees (from Wikipedia).

So, any line in a plane should divide a plane into two parts, each part is half of the rotation so a straight line froms an angle of $180^\circ$.

Answer 3

Total $ 360^{\circ} $ at a point is divided exactly into two halves; so by definition a straight line without direction change ( a shoot through) is a straight angle.