# Anyone have revision for finding the equation of a perpendicular line to another line on a graph?

I really don't get how to work out the intercept of a perpendicular line on graphs - especially when they give you two sets of coordinates. If anyone has any questions and could show me how you work them out that would be really helpful thanks =)

Posted Thu 7th June, 2012 @ 20:35 by Joanne

Perpendicular basically means at right angles to something, so the gradients of two lines would multiply together to make -1.

For example, if you were given the equation y= 2x +1, where the gradient is 2

The perpendicular gradient would be -1/2

Because 2 x -1/2 = -1

So say if you were given the question: line passes through co ordinates (0,5) and (7,9)
And were asked to find the perpendicular to this line

1) First find the gradient of the original line given: difference in y / difference in x

so gradient is 9-5 / 7-0 = 4/7

So then at point (0,5), your equation of the line would be y= -7/4x +5

:D hope that helps **

Answered Thu 7th June, 2012 @ 20:47 by Terrie

well joanne !!

u know the equation for a straight line is y=mx+c

n if u work out the gradient  u will get (let say the gradient is 5)   y=5x + c

sub in the x value and make c the subject of the formula

Answered Thu 7th June, 2012 @ 20:58 by Braniac

Yes thanks, but what would you do if you were given coordinates that didn't include (0,something)? And you had to find the intercept?

Answered Thu 7th June, 2012 @ 20:53 by Joanne

Beelal - but you don't know what y is so you would have two unknowns? Can you give me an example of what you mean please?

Answered Thu 7th June, 2012 @ 21:04 by Joanne

I think what beelal is trying to say is that using y=mx+c you could substitute any coordinate that you know is on the perpendicular line into the equation because you would know the y and the x coordinate and so can find the c (constant). :)

Answered Thu 7th June, 2012 @ 22:21 by Dilly

yes soz i left it too vague didnt i but wot dilly said is right. Just sub in a co-ordinate which the perpendicular line crosses so if we say the point (3,7, just sub it into y=mx + c n u can make the subject of the formula!!!

P.S if u didnt know the y co-ordinate just sub it into the original curve or line n u will get the y co-ordinate

Answered Thu 7th June, 2012 @ 22:46 by Braniac

sorry what is 'y' and what is 'x' again?????

Answered Thu 7th June, 2012 @ 23:34 by anonymous

y is the function n also a co-ordinate value can be subbed in as well. X is the coordinate

Answered Thu 7th June, 2012 @ 23:56 by Braniac