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Horizontal and vertical lines, x + y = k and xy = k
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Learn it $y = k$ is a horizontal line and $x = k$ is a vertical line. $x + y = k$ is a slanted line with slope −1. $xy = k$ is a curve, not a line.
Example: Describe $x = -3$.
- Every point has x-coordinate −3: (−3, 0), (−3, 5), …
- It is a vertical line through −3 on the x-axis.
- Its slope is undefined.
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