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Horizontal Or Vertical Line
Definition: A horizontal line i passing through k on the y-axis is precisely a plot of all solutions (x, y) of the equation y = k; i.e., i is the graph of y = k. A vertical line m passing through h on the x-axis is precisely a plot of all solutions (x, y) of the equation x = h; i.e., m is the graph of x = h.
Source: "Precalculus" by David H. Collingwood and K. David Prince at the University of Washington
Example: A horizontal line that is 3 units above the x-axis is written as the equation y=3. A vertical line that is 5 units to the left of the y-axis is written as the equation x=-5.