1. The problem is to understand what perpendicular lines are. 2. Perpendicular lines are two lines that intersect at a right angle, which means they meet at an angle of 90 degrees. 3. The key formula to check if two lines are perpendicular is based on their slopes. If the slope of one line is $m_1$ and the slope of the other line is $m_2$, then the lines are perpendicular if: $$m_1 \times m_2 = -1$$ 4. This means the slopes are negative reciprocals of each other. For example, if one line has slope 2, the other must have slope $-\frac{1}{2}$. 5. If one line is vertical (slope undefined), the other must be horizontal (slope 0) to be perpendicular. 6. Understanding this helps in geometry, algebra, and coordinate plane problems involving angles and lines.