## Intermediate Algebra for College Students (7th Edition)

RECALL: (i) The slope, $m$, of the line that passes through the points $(x_1, y_1)$ and $(x_2, y_2)$ is given by the formula: $m=\dfrac{y_2-y_1}{x_2-x_1}$ (ii) A line whose slope is positive rises from left to right. (iii) A line whose slope is negative falls from left to right. (iv) A horizontal line has a slope of zero. (v) A vertical line has an undefined slope. Use the formula in (i) above to find the slope of the line that passes through the points $(a, b)$ and $(a, b+c)$ to obtain: $m=\dfrac{(b+c) -b }{a-a}=\dfrac{c}{0}$ (slope is undefined) The slope is undefined, so the line is vertical.