I need point/slope formula help!

Mathematics
Tutor: None Selected Time limit: 1 Day

I am preparing for the NY State Geometry Regent, and I answered a multiple-choice point/slope question. The question reads: What is an equation of the line that passes through (-9,12) and is perpendicular to the line whose equation is y = 1/3x + 6?

The correct answer was: y = -3x - 15.

I went over the math once again after seeing the correct answer. I still got y = 1/3x + 15. I don't see how the answer is y = -3x - 15. Can someone please explain the problem to me?

Jun 18th, 2015

Thank you for the opportunity to help you with your question!

Thank you for the opportunity to help you with your question!

The given line is y = 1/3 x + 6
let the slope of the line be m1, comparing with the standard form y=mx+c
we get, m1= 1/3

let the slope of the required line be m2
since both the lines are perpendicular to each other, m1*m2 =-1
so, 1/3*m2 = -1
m2 =-3

so, the required equation will be y= -3x + c

since this line passes through (-9,12)
we get 12 = -3*-9 + c
12 = 27 +c
c = 12-27 = -15

so, the required equation is y = -3x -15

Please let me know incase you have any doubts :)

Please appreciate my help and select my answer as the best! Thanks :) Please appreciate my help and select my answer as the best! Thanks :)
Jun 18th, 2015

You said: Since both of the lines are perpendicular to each other, m1 * m2 = -1.  Where did this -1 come from?


Jun 18th, 2015

this is the basic formula.

If lines are parallel then m1 = m2
if lines are perpendicular then m1 * m2 = -1

Please check your text book, it would be mentioned somewhere. Let me know if you still have any doubt.

Jun 18th, 2015

Are you studying on the go? Check out our FREE app and post questions on the fly!
Download on the
App Store
...
Jun 18th, 2015
...
Jun 18th, 2015
Dec 6th, 2016
check_circle
Mark as Final Answer
check_circle
Unmark as Final Answer
check_circle
Final Answer

Secure Information

Content will be erased after question is completed.

check_circle
Final Answer