# Are these two lines perpendicular?

Asked by MyNewtBoobs (19026) September 26th, 2010

Now, before you say “homework question”, let me explain. I already did it, and I keep coming up with “yes”, but the computer says “no”. But I don’t know what I’m doing wrong.

4x-y=8
x-4y=-5

Observing members: 0 Composing members: 0

They’re not. Just draw them on a piece of paper and it’ll appear clear as day.

The coefficient of x has to be n in one and -(1/n) in the other for them to be perpendicular, in this case the second one is 1/n.

Thammuz (9267)

@Thammuz Crap. Ok, then I just have to figure out where I got the – from. Thanks.

MyNewtBoobs (19026)

@papayalily You probably jumbled the signs up when moving one of the terms from one side of the = to the other, happens all the time.

The equations end up “y = 4x-8” and “y=¼*x+5/4”

Thammuz (9267)

Stated another way too, if the product of two slopes is -1, they are perpendicular.

cockswain (15254)

4x – y = 8
4x = 8 + y
y = 4x – 8
m = 4

x – 4y = -5
-4y = -x – 5
y = ¼x + 5/4
m = ¼

They’re not perpendicular because neither slope is the negative reciprocal of the other.

Just in the way of moral support, because I’m mathematically impaired… my mother was a mathematician and she used to say that one thing she frequently had to remind herself and stress to her students so they wouldn’t fret so much was that if you were having difficulty with a problem or formula more likely than not it wasn’t the higher level math that you were doing wrong but that somewhere along the line you had added 2 + 2 and come up with 5. So just don’t forget to pay attention to the little details.

lillycoyote (24810)

Absolutely. Or switched a + to a -

cockswain (15254)

@lillycoyote At some point, I made ¼ into -¼. I’m not really concerned, since I understand the basic process. It’s a new program, so there was that moment of like “omg, what if the program’s wrong, and I’m going to fail my math test tomorrow because of it”.

MyNewtBoobs (19026)

@papayalily No, I don’t think you should be concerned. You’ll do fine on your test.

lillycoyote (24810)

or