Algebra 1 / Unit 5: Linear Functions
5.4 Parallel and Perpendicular Lines
Notes
- Parallel lines have the same slope and different $y$-intercepts. They never meet.
- Perpendicular lines meet at a right angle. Their slopes are negative reciprocals: $m_1 \cdot m_2 = -1$. A slope of 2 pairs with $\displaystyle -\frac{1}{2}$.
- A horizontal line and a vertical line are perpendicular.
Practice
Which line is parallel to $y = 5x - 2$?
Same slope, 5.
The slope of a line perpendicular to $\displaystyle y = -\frac{2}{3}x + 1$ is:
Flip and change the sign.
Write the line perpendicular to $y = 2x$ through $(4, 1)$.
Slope $\displaystyle -\frac{1}{2}$; $\displaystyle 1 = -\frac{1}{2}(4) + b$ gives $b = 3$.
Ready for more? Take the Unit 5 test.