# Thread: ´PArallel perpedicular or neither

1. ## ´PArallel perpedicular or neither

Y=-2+5
Y=1/2x+4

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

2. 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

3. 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?

4. 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

5. 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