Math CentralQuandaries & Queries


Question from cindy, a student:

How do I graph y=-2x-1?

Hi Cindy,

Since it is a linear equation I know the graph is a straight line so I only need to plot two points and then draw the line through these points. You get to choose the points so chose them so that the arithmetic is easy. I would first take $x = 0$ and substitute in the equation to get

\[y = -2 \times 0 - 1 = -1\]

and hence the point $(0, -1)$ is on the graph. Now choose another $x$ value, say $x = 1$ or $x = 2$ or $x = -1$ or some other value, substitute into the equation to obtain a second point on the graph. Graph this point also. Since the graph is a line use a ruler to draw the line that lies on these two points.

If you don't recognize that the graph is a line you will need to plot more than two points. If you plot 4 or 5 points reasonably accurately then using a ruler you will see that the points lie on a line.


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