The phrase "linear equation" takes its origin in this correspondence between lines and equations: a linear equation in two variables is an equation whose solutions form a line.

If , the line is the graph of the function of that has been defined in the preceding section. If , the line is a vertical line (that is a line parallel to the -axis) of equation which is not the graph of a function of .

Similarly, if , the line is the graph of a function of , and, if , one has a horizontal

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