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## #1 2011-11-25 17:49:48

Member
Registered: 2011-05-11
Posts: 171

### Sufficient condition left out about parallel lines

From:

http://www.mathsisfun.com/algebra/line-parallel-perpendicular.html

"Parallel Lines

How do you know if two lines are parallel?

Their slopes are the same!"

The lines must also have different y-intercepts, else they are the
same line.

So, if you know that their slopes are the same, then you don't
(automatically) know if they are parallel lines or not.

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Edit:

On parallel vertical lines:

"a vertical line is parallel to another vertical line."

No, for example, x = -2 is a vertical line, and

3x + 6 = 0 is another vertical line.  These lines
are not parallel to each other because they are the
same line.

Distinct vertical lines are parallel to each other.

Last edited by reconsideryouranswer (2011-11-25 18:01:32)

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## #2 2011-11-25 23:28:25

MathsIsFun
Registered: 2005-01-21
Posts: 7,651

### Re: Sufficient condition left out about parallel lines

Thank you. Will review that page.

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

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## #3 2012-05-25 10:46:04

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: Sufficient condition left out about parallel lines

Actually,tge definition of when two lines are parallel is when they are in the same plane and they do not intersect. I.e. A line is always parallel to itself.

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