# ´PArallel perpedicular or neither

• Aug 22nd 2007, 05:52 AM
ChE
´PArallel perpedicular or neither
Y=-2+5
Y=1/2x+4

how do i know if the lines are parallel perpendicular or neither?
• Aug 22nd 2007, 06:40 AM
topsquark
Quote:

Originally Posted by ChE
Y=-2+5
Y=1/2x+4

how do i know if the lines are parallel perpendicular or neither?

Two lines are parallel if they have the same slope.
Two lines are perpendicular if the slopes obey: $m_1 m_2 = -1$.

-Dan
• Aug 23rd 2007, 02:45 PM
kwah
If the equations have the same gradient, then it is parallel.
ex.
$y = 2x + 1$
$y = 2x - 13$
the + or - just tells you how far up/down the y axis it is

if, on the other hand, they are the same but the negative equivalent, then they are parallel
ex.
$y = 2x + 1$
$y = -2x - 13$
again, the + or - number doesnt matter; for example, $+0.5x$ & $-0.5x$, $+20,000x$ & $-20,000$, and $+zx$ & $-zx$ are all perpendicular to each other

does this make sense?
• Aug 23rd 2007, 02:48 PM
topsquark
Quote:

Originally Posted by kwah
if, on the other hand, they are the same but the negative equivalent, then they are parallel
ex.
$y = 2x + 1$
$y = -2x - 13$
again, the + or - number doesnt matter; for example, $+0.5x$ & $-0.5x$, $+20,000x$ & $-20,000$, and $+zx$ & $-zx$ are all perpendicular to each other

This is not true. The lines
$y = mx + b_1$
$y = -mx + b_2$
are not parallel (unless m happens to equal 0.) Nor are they perpendicular (unless m happens to equal 1.)

-Dan
• Aug 23rd 2007, 03:51 PM
kwah
Quote:

Originally Posted by topsquark
This is not true. The lines
$y = mx + b_1$
$y = -mx + b_2$
are not parallel (unless m happens to equal 0.) Nor are they perpendicular (unless m happens to equal 1.)

-Dan

sorry i meant perpendicular with the value m=1 in mind .. my mistake ..

you are right (as always) and i am wrong (as always) in that, when the values of m are multiplied, they result in a product of -1