A Right-Angle Line

To draw a perpendicular line through a given point, first find the perpendicular slope. Then draw the line through the point with that slope, the same way you draw any line from a point and a slope. The slope of $y = \frac{1}{2}x+1$ is $\frac{1}{2}$, so the perpendicular slope is -2. The perpendicular line through (0, -1) is $y = -2x-1$.

The two lines cross once, at a right angle. The first line rises $\frac{1}{2}$ for each step to the right. The second line falls 2 for each step to the right. The product of the two slopes is $\frac{1}{2}\cdot \left(-2\right) = -1$.