Skill: Use supplementary angles informally

When two adjacent angles form a straight line, together they make a straight angle of $180^\circ$. This means they are supplementary angles.

To find a missing angle:

1. Start with $180^\circ$.
2. Subtract the angle you know.
3. Label the result with degrees.

Example: Angles $\angle AOB$ and $\angle BOC$ form a straight line. If $\angle AOB = 65^\circ$, find $\angle BOC$.

Since the angles form a straight line, their measures total $180^\circ$:

$$65^\circ + \angle BOC = 180^\circ$$

Subtract $65^\circ$ from $180^\circ$:

$$\angle BOC = 180^\circ - 65^\circ = 115^\circ$$

So, $\boxed{\angle BOC = 115^\circ}$.

Remember: supplementary angles total $180^\circ$. This is different from complementary angles, which total $90^\circ$. Also, two angles that merely share a vertex are not necessarily supplementary; they must be adjacent and form a straight line.