# Equation of a straight line

• June 30th 2011, 12:37 PM
Googl
Equation of a straight line
Is it possible to find the equation of a straight line given two coordinates of that line.

I have a problem I need to solve involves finding the area between a curve and a straight. But I am given only the equation of the curve, I have managed to find two coordinates of the line from differentiating the curve. But I need to know the equation of the line to differentiate it.
• June 30th 2011, 12:55 PM
skeeter
Re: Equation of a straight line
Quote:

Originally Posted by Googl
Is it possible to find the equation of a straight line given two coordinates of that line.

I have a problem I need to solve involves finding the area between a curve and a straight. But I am given only the equation of the curve, I have managed to find two coordinates of the line from differentiating the curve. But I need to know the equation of the line to differentiate it.

yes ... find the slope between the two points and determine the equation using either one of the two points, the slope between them, and the point-slope form of a linear equation.
• June 30th 2011, 01:06 PM
elieh
Re: Equation of a straight line
Find the gradient first,

Then using y=mx+c:
Plug in one of the co-ordinates you have into the equation to find c.
• June 30th 2011, 01:10 PM
Googl
Re: Equation of a straight line
Yes, that's what I have found. But when I try different points the c value changes. Here are some points and the straight line:

y = 12x + c

Points (2, 16) (10/3, 0)
• June 30th 2011, 01:18 PM
mr fantastic
Re: Equation of a straight line
Quote:

Originally Posted by Googl
Yes, that's what I have found. But when I try different points the c value changes. Mr F says: Surely it is obvious that his statement is absolutely useless unless you show your working, including how you got 12 as the gradient.

Here are some points and the straight line:

y = 12x + c

Points (2, 16) (10/3, 0)

..
• June 30th 2011, 01:18 PM
Googl
Re: Equation of a straight line
Hi,

Ignore this. I have found the error. 12 is a negative.

Thanks.
• June 30th 2011, 01:21 PM
elieh
Re: Equation of a straight line
Use $y-y'=m(x-x')$