# Slope-intercept

• Mar 28th 2012, 06:27 PM
Kibbygirl
Slope-intercept
Problem states -
Find the slope-intercept form of the equation of the line perpendicular to 4x+5y= -11, and containing the point (-7,6):

I came up with y= - 4/5x + 2/5

Is this right? I feel like the example I am following from the book is not the way the instructor was telling the class. I am not sure if I am even understanding the book example. I'm not sure if I am doing the right steps at all.
• Mar 28th 2012, 06:55 PM
Prove It
Re: Slope-intercept
Quote:

Originally Posted by Kibbygirl
Problem states -
Find the slope-intercept form of the equation of the line perpendicular to 4x+5y= -11, and containing the point (-7,6):

I came up with y= - 4/5x + 2/5

Is this right? I feel like the example I am following from the book is not the way the instructor was telling the class. I am not sure if I am even understanding the book example. I'm not sure if I am doing the right steps at all.

How did you get 2/5 for the y intercept?
• Mar 28th 2012, 07:28 PM
Kibbygirl
Re: Slope-intercept
I'm unsure if I did it right..
I did:

y-6= -4/5(x-[-7])
y-6 = -4/5x - 28/5
y= -4/5x - (-2/5)
y= -4/5 + 2/5
• Mar 28th 2012, 08:12 PM
Prove It
Re: Slope-intercept
Quote:

Originally Posted by Kibbygirl
I'm unsure if I did it right..
I did:

y-6= -4/5(x-[-7])
y-6 = -4/5x - 28/5
y= -4/5x - (-2/5)
y= -4/5 + 2/5

It's easier to just transpose the equation of the line you have been given.

\displaystyle \displaystyle \begin{align*} 4x + 5y &= -11 \\ 5y &= -4x - 11 \\ y &= \frac{-4}{5}x - \frac{11}{5} \end{align*}

What are the gradient and the y-intercept?
• Mar 28th 2012, 08:57 PM
Kibbygirl
Re: Slope-intercept
- 4/5 and 11/5?
• Mar 28th 2012, 09:45 PM
biffboy
Re: Slope-intercept
The gradient of the line you have been given is -4/5 so the line perpendicular to this has gradient +5/4. So you want the line with gradient +5/4 through the point (-7,6)
• Mar 28th 2012, 10:30 PM
Kibbygirl
Re: Slope-intercept
So would the final answer be y= 5/4x+2?
• Mar 28th 2012, 11:04 PM
biffboy
Re: Slope-intercept
Not +2. Line is y=5/4x+c. (-7,6) has to be on line. So we need 6=-35/4+c. So 59/4=c
Line is y=5/4x+59/4
Multiply both sides by 4
4y=5x+59 (any other arrangement of this is OK)