y = mx + b in words
m is the change in y for each 1-unit increase in x. b is the y-value at x = 0. A fixed fee plus a per-use charge (rent + per mile, membership + per class) often fits this form.
Example
Phone plan: $20 flat plus $0.10 per minute. Minutes are x, cost is y: y = 0.10x + 20. Ten minutes: $21. One hundred: $30.
See the line
Mistakes
- Calling every graph a line without checking points.
- Confusing flat fee (b) with rate (m).
Use the calculator
FAQ
- When is it not linear?
If equal steps in x do not produce equal steps in y, the graph is not a single straight line over that interval—use the right model (curve, exponential, etc.) for that data.