Cambridge Additional Mathematics

178 Straight line graphs (Chapter 7)

FINDING THE EQUATION OF A LINE

In order to find the equation of a line, we need to know some information.

Suppose we know the gradient of the line is 2 , and that the line

passes through (4,1).

We suppose (x,y) is any point on the line.

The gradient between (4,1) and (x,y) is y¡^1

x¡ 4

, and this

gradient must equal 2.

So,

y¡ 1

x¡ 4

=2

) y¡1=2(x¡4) fmultiplying both sides by (x¡4)g

) y¡1=2x¡ 8 fexpanding the bracketsg

) y=2x¡ 7 fadding 1 to both sidesg

This is the equation of the line in gradient-intercept form.

We can find the equation of a line if we know:

² itsgradientand thecoordinates of any pointon the line, or

² thecoordinates of two distinct pointson the line.

If a straight line has gradientmand passes through the point (x 1 ,y 1 )

then its equation is

y¡y 1

x¡x 1

=m or y¡y 1 =m(x¡x 1 ).

We can rearrange this equation into either gradient-intercept or general form.

Example 1 Self Tutor

Find, ingradient-intercept form, the equation of the line through

(¡ 1 ,3) with a gradient of 5.

The equation of the line is y¡3=5(x¡(¡1))

) y¡3=5x+5

) y=5x+8

Example 2 Self Tutor

Find, ingeneral form, the equation of the line with gradient^34 which

passes through (5,¡2).

The equation of the line is y¡(¡2) =^34 (x¡5)

) 4(y+2)=3(x¡5)

) 4 y+8=3x¡ 15

) 3 x¡ 4 y=23

y

x

(4 1),

(x y),

gradient=2

O

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Additional Mathematics
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