OG DS Line equation question : DS Archive
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# OG DS Line equation question

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OG DS Line equation question [#permalink]

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27 Sep 2006, 10:48
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Hi

Check this DS question from the OG:

The quiz:
If I have one point A that satifisfies the equation K
for example (-7,-19) for Y=3x +2, is it sufficient to determin whether Line K passes by point A?

cheers
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27 Sep 2006, 19:01
lan583 wrote:

Yep, points (1, 5) are the only two points that satisfy both equations
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28 Sep 2006, 08:09
I think it is E..

From both the equations, it is possible that 3r +2 -s is not equal to 0 in which case r,s is not a point on the line..

What is the OA?
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28 Sep 2006, 12:15
OA is C

Congratulations
solving each equation from each statement, I get points (-7,-19) and (8,26)

Combining St1 and St2 I get y=3x +2

How did you get point (1,5)?

thanks
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28 Sep 2006, 14:34
GMATT73......WOULD YOU PL EXPLAIN HOW WE GOT THE C

IN MORE DETAILS MAN ...I VE GOT A PERSONAL PROBLEM WITH COORDINATE GEOMETRY

HELP
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OG DS Line equation question [#permalink]

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28 Sep 2006, 16:01
Ok I went with this question a really long way so DON'T DO that haha!

I basically opened up the terms for both the equations and subtracted the 2nd equation from the first, lot of terms cancel out. In the end, you get an equation of form

s=3r+2 which is the same as eq in the question.

But however , I'd like to know what the short cut is Matt can you please provide your explaination how you got your answer coz I seriously can't spend so much time like this on the exam
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Re: OG DS Line equation question [#permalink]

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28 Sep 2006, 21:01
Does y=3x+2 contain (r,s)?

It is C.
Statement 1: (3r+2-s)(4r+9-s)=0
So either (3r+2-s) = 0 or (4r+9-s)=0
If 3r+2-s =0
ie s= 3r+2 which mean y=3x+2 contain (r,s)

But if (4r+9-s) =0 then y=3x+2 does not contain (r,s).

So (1) is not sufficient.

Statement 2: (4r-6-s)(3r+2-s) = 0
Thinking along the same lines (2) is not sufficient.

If we combine both (3r+2-s) must be 0
Hence the line contains (r,s)
So it is C
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Last edited by cicerone on 25 Sep 2008, 00:11, edited 1 time in total.
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29 Sep 2006, 10:48
It's clear...

Thanks

However you need some imagination to get to this conclusion
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