September 8th, 2010, 07:05 AM  #1 
Member Joined: Apr 2010 Posts: 91 Thanks: 0  Vectorsintersection of lines
Find the intersection of the lines: and I tried equating the x, y and z coordinates of both lines, but when I try to solve for lambda in each I dont get the same answer, meaning they dont intersect. However the answer to this problem was the line: . I dont understand how "a line" could be the answer. 
September 8th, 2010, 07:34 AM  #2 
Global Moderator Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4  Re: Vectorsintersection of lines
There's definitely something wrong here!

September 8th, 2010, 03:51 PM  #3  
Global Moderator Joined: May 2007 Posts: 6,710 Thanks: 675  Re: Vectorsintersection of lines Quote:
7+2u=34v (x equality) 177u=3+14v (z equality) Solve for u and v to get point of intersection.  
September 8th, 2010, 08:29 PM  #4  
Math Team Joined: Dec 2006 From: Lexington, MA Posts: 3,267 Thanks: 408  Re: Vectorsintersection of lines Hello, TsAmE! [color=green]mathman[/color] is correct . . . Two parameters should have been used. Quote: [color=beige]. . [/color] [color=beige]. . [/color]  
September 9th, 2010, 02:34 AM  #5 
Member Joined: Apr 2010 Posts: 91 Thanks: 0  Re: Vectorsintersection of lines
I am a bit confused. Even though the 2 lines are parallel, howcome Q lies on the other line? Isnt it possible that if L1 is parallel to L2, Q might not lie on L1 (like this in attachment)?

September 9th, 2010, 05:15 AM  #6 
Global Moderator Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4  Re: Vectorsintersection of lines
It is possible that parallel lines are not the same. But once we determined that they have (at least) ONE point in common, then they must have ALL points in common. 
September 9th, 2010, 02:03 PM  #7 
Member Joined: Apr 2010 Posts: 91 Thanks: 0  Re: Vectorsintersection of lines
Oh ok. How would the answer: represent that these lines lie on each other?

September 9th, 2010, 02:22 PM  #8 
Global Moderator Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4  Re: Vectorsintersection of lines
The format of the answer tells us about the question. If two or more lines cross (touch) in a single point, they intersect and the point is the solution to the system of equations. If two lines DON'T cross, then they are parallel (or skew, in higher dimensions). If two lines cross EVERYWHERE, then they are the same line. You can conclude that lines with the same slope through a common point are actually the same line. All that to say... if the ANSWER is a line and the QUESTION is about two lines, then the "two" lines are actually just the same line. 

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