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Equation of a Straight Line


The equation of a straight line is usually written this way:

y = mx + b

(or "y = mx + c" in the UK see below)

What does it stand for?

 
Equation of a Straight Line
Slope (or Gradient) Y Intercept

 

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



m  = 
2
1
 =  2

b = 1

Therefore y = 2x + 1


Example 2

m  = 
3
-1
 =  –3

b = 0

This gives us y = –3x + 0
We do not need the zero!

Therefore y = –3x

 

Play With It

You can see the effect of different values of m (the slope) and b (the y intercept) at Explore the Straight Line Graph

 

Country Note:

Different Countries teach different "notation":

In the US the notation is: y = mx + b
In the UK, Australia, Ireland, India, Jamaica, Pakistan, Brunei, Malaysia and Malta: y = mx + c
In Denmark and Holland: y = ax + b
In Finland and Ukraine: y = kx + b
In Sweden: y = kx + m
   
In your country: let us know!

... but it all means the same thing, just different letters.