Name: Writing equations of perpendicular lines
Uploaded: 2015-05-04T18:20:58Z
Description: Given line A and point P, Sal finds the equation of the line perpendicular to A that passes through P.

Question 1

My question is, how do you find the intersecting point of the perpendicular lines if they do not intercept on the y axis? WITHOUT USING A GRAPH. I know this was touched on already but I think I am asking a slightly different question.

For example,  Find the distance between the point (2,6) and the line y=−2x.

This tells me that the slope of the perpendicular line is 1/2. The equation for my second line would then be, (6) = 1/2*(2) + b. Solving this gives me the y-intercept of 5.

My two points for my second line are now (2,6) (0,5). By using the graph(which is what I am trying to avoid), I can find that the intersecting point of both lines is (-2,4)

Then, using the "distance formula" *cough*(pythagorean theorem)*cough* for the two points (2,6) and (-2,4) ==> (2-(-2))squared + (6-4)squared ==> 16+4 ==> square root of 20 being 2 root 5. So, I have the answer BY USING A GRAPH.

My question is, how do you find the intersecting point of the perpendicular lines if they do not intercept on the y axis? WITHOUT USING A GRAPH

What formula would be used? What if I end up not having graph paper? What if my line is screwy? I feel that having an equation to figure this out would let me know that I understand fully and NOT have to rely on NEEDING a graph to solve. Thanks for any help!

Accepted Answer

Instead of finding the y-intercept of the point (2,6), *find the complete equation of the line*. Do this by doing what you did before--find your m and b values.

You know that the line of (2,6) is perpendicular to y=-2x, so m=1/2. You already solved for b after substituting in the values of m, x and y:

y=mx+b
(6)=(1/2)(2)+b
6-1=b, *b=5*

But instead of finding your y intercept, write down the general equation of the line! Since you know m and b, the equation of this line is y=1/2x+5.

Now what? To find where y=1/2x+5 and the original line y=-2x intersect, *set them equal to each other*. Let y in both of the equations equal the same value. You are doing this because at the two lines' point of intersection, both lines will share the same x and y value.

So, let y=1/2x+5 equal y=-2x. That means

-2x = 1/2x+5
0= 5/2x +5
-5 = 5/2x
-2 =x

Now you now that at the point where the two lines intersect, x=-2. You can substitute this x value into *either of the two original equations* to obtain the corresponding y value. Eg.

Sub x=-2 into y=-2x

y=-2(-2)
y=4

OR sub x=-2 into y=1/2x+5

y=(1/2)(-2)+5
=-1 + 5
y=4

Note that in both equations, you get the same y value.

So now you know that the point of intersection is (-2,4), without graphing. I'm sure there's a video on this on the site; try looking up "substitution" or "elimination".

Question 2

how would you solve this?

Determine the equation of the line that is perpendicular to y= 3x+1 and has the same y-intercept.

Accepted Answer

𝑦 = 3𝑥 + 1
All perpendicular lines have negative reciprocal slopes of each other. Therefore, the slope of our perpendicular line would be -1/3. So our equation for our perpendicular line so far is:
𝑦 = (-1/3)𝑥 + 𝑏
𝑏 in this equation is simply the 𝑦-intercept. We are told that the perpendicular line has the same 𝑦-intercept as the original line so that means we have 𝑏 = 1. Therefore, our finally equation for our perpendicular line is:
𝑦 = (-1/3)𝑥 + 1
Comment if you have questions.

Question 3

1:04 Why is 1/2 the inverse of 2? How do you figure out the inverse of a number?

Accepted Answer

Hey Nate, good question! 
As per the introductory video found here https://www.khanacademy.org/math/geometry/hs-geo-analytic-geometry/modal/v/parallel-and-perpendicular-lines-intro a line is perpendicular if the slope is the reciprocal inverse of the first line.

Some examples:

3 x = - 1/3 x
9 x = - 1/9 x
15 x = - 1/15 x
1/15 x = -15 x
1/9 x = - 9 x
1/3 x = -3x
x = - x

If you get unsure about the last one, think of it as 1/1 x =  -1/1 x as 1/1 is just 1 and 1 * x is just x it is then simplified to x = -x

Same thing goes for 15x = - 1/15 x just think of the number in front of the x in terms of a fraction, as in 15 turns into 15/1 x = - 1/15x to make it a little easier.

If you want to know more details, check out this article https://en.wikipedia.org/wiki/Multiplicative_inverse

Or this lesson plan on Khan Academy shows you what an inverse of a fraction is:
https://www.khanacademy.org/math/pre-algebra/pre-algebra-arith-prop/pre-algebra-arithmetic-properties/v/inverse-property-of-multiplication

Question 4

how do you find the equivalence in slopes

Accepted Answer

when you have the slope for let's say line A
- then a line parallel to line A will have a slope equal to the slope of line A
- a line perpendicular to line A will have a slope equal to the negative inverse of the slope of line A

for example, if slope = 2
then the negative inverse is -1/2

Question 5

Is the sign also going to be inversed?

Accepted Answer

Two things, perpendicular slopes have opposite (signs) reciprocal (multiplicative inverse).  So if you have a slope of -3/2, the sign is changed to positive 2/3, no sign to invert.  If you have 3/2, the answer is -2/3, the negative sign is added, but there is no sign to invert either.  Secondly, negatives drift to the top, so even if you were not talking about perpendicular lines, if you want the multiplicative inverse of -2/3, you could say 3/-2,but then the negative would drift to the top to get -3/2.

Question 6

what about fractions? my equation; line a is y= -4/5x-19 with line b having points (4,-3) and the two are perpendicular

Accepted Answer

Perpendicular lines have opposite (signs) reciprocal (flip fraction).  So if slope is -4/5, the perpendicular slope is 5/4.  If y=mx+b, we can subtract mx on both sides to get b=y-mx, and subbing in x=4, y=-3, and m=5/4, we get b=-3-(5/4)(4), 4s cancel, so you have b=-3-5=-8.  Thus equation is y=5/4 x -8.

Question 7

How do I find the negative inverse of 1?

Accepted Answer

Write your number as a fraction then flip the fraction and flip the sign. Hence, 1 could be written as 1/1. Flipping the fraction keeps it the same as 1/1. Change the positive sign to a negative sign, so it's -1/1 or -1. Here's another example. If you have a slope of -1/4, flip it to -4/1 then change the sign, making the perpendicular slope 4/1 or 4.

Question 8

I've been working out the follow up problems to this video and I'm stuck at finding the distance between (0,-8) and x = 5. What do I do when I get an equation that is not clearly y = mx + b?

Accepted Answer

Hi Alicia,
x=5 means that for all values of y, x = 5. So on the graph, there will be a vertical line at x=5. To figure out the distance, then, from (0,-8) to x=5, draw a perpendicular line towards x=5. In this case, that line will be horizontal and the lines will meet at (5,-8). Since your x value at (0,-8) is 0, and your x value at the intersection (5,-8) is 5, and your y value remains the same, the distance between (0,-8) and x=5 is 5.
Hope that helps.

Course: High school geometry > Unit 6

Writing equations of perpendicular lines

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