Sizu Riatto
Algebra · Chapter 11

Plot Points and Read a Table

A point is an address: across first, then up. A rule like y = 2x + 1 turns a table of numbers into a line of points.

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Opens on Chapter 11’s example: the four points A to D on the first grid, the rule y = 2x + 1, and the points (2, 3) and (8, 7).

1. Points to plot
2. A rule y = mx + b
3. Two points
Enter your numbers, then press Work it out.

How it works

The first number of a point says how far across from the center (left if negative), the second how far up (down if negative). A rule y = mx + b makes a table: put each x in, work out y, and the points line up on a straight line. Reading backward (which x gives y = 9?) is solving 2x + 1 = 9. Two points on the same row are as far apart as the gap between their x’s; on the same column, the gap between their y’s. The midpoint is the average of the x’s and the average of the y’s.

From the book, Chapter 11. y = 2x + 1 gives 1, 3, 5, 7 and 9 for x = 0 to 4, and y = 9 at x = 4. The midpoint of (2, 3) and (8, 7) is (5, 5); (−2, 1) to (4, 1) is 6 across; (1, −3) to (1, 5) is 8 up. Every chapter and its calculator →

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