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**reconsideryouranswer****Member**- Registered: 2011-05-11
- Posts: 172

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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I wish a had a more interesting signature line.

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,549

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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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,594

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.

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

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