Equation of a Straight Line
The equation of a straight line is usually written this way:
(or "y = mx + c" in the UK see
below)
What does it stand for?
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y = how far up
x = how far along
m = Slope or Gradient (how steep the line is)
b = the Y Intercept (where the line crosses the Y axis) |
Knowing this we can work out the equation of a straight line:
Example 1

b = 1 (where the line crosses the Y-Axis)
Example 2
b = 0
This gives us y = 3x + 0
We do not need the zero!
Example 3: Vertical Line
What is the equation for a vertical line?
The slope is undefined ... and where does it cross the Y-Axis?
In fact, this is a special case, and you use a different equation, not "y=...", but instead you use "x=...", like this:
Other Forms
What we have been looking at is called the "slope-intercept" form.
The equation of a straight line can be written in other ways. Another popular form is:
And you can read more about it at Point-Slope Equation of a Straight Line
Footnotes
Country
Note:
Different Countries teach different "notation" (as sent to me by kind readers):
| In the US and Canada the notation is: |
y = mx + b |
| In the UK, Australia, Bahamas, Brunei, India, Indonesia, Ireland, Jamaica, Malaysia, Malta, Nepal, Pakistan, and South Africa: |
y = mx + c |
| In Brazil, Denmark, Holland and Viet Nam: |
y = ax + b |
| In China, Finland and Ukraine: |
y = kx + b |
In Greece: |
ψ = αχ + β |
| In Italy: |
y = mx + q |
| In Japan: |
y = mx + d |
| In Latvia: |
y = jx + t |
| In Romania: |
y = gA + C |
| In Sweden: |
y = kx + m |
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| In your country: |
let us know! |
... but it all means the same thing, just different letters.
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