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In the xy-plane, what is the y-intercept

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dtriffonov
 Post subject: In the xy-plane, what is the y-intercept
 Post Posted: Tue Sep 15, 2009 7:58 am 
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Students


Posts: 4
In the xy-plane, what is the y-intercept of line L?

(1) The slope of line L is 3 times its y-intercept?
(2) The x-intercept of line L is (-1/3)

walk me through the solution please. Thanks,
Daniel
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nitin_prakash_khanna
 Post subject: Re: In the xy-plane, what is the y-intercept
 Post Posted: Tue Sep 15, 2009 1:59 pm 
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Students


Posts: 68
let the line be
y = mx+c

whats the y intercept, i.e when x = 0 , y=c
So question is asking what's c?

St 1 -> Slope which m = 3 c.
Doesnt help so insufficient.

St 2 -> x intercept which when y = 0, x = -c / m = -1/3
which gives m = 3c
not much help. so insufficient.

Combine both all we get is m= 3c
Nothing new.

So Ans E.
Hope its the OA
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dtriffonov
 Post subject: Re: In the xy-plane, what is the y-intercept
 Post Posted: Wed Sep 16, 2009 2:29 am 
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Students


Posts: 4
thank you.
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RonPurewal
 Post subject: Re: In the xy-plane, what is the y-intercept
 Post Posted: Sat Sep 26, 2009 2:19 am 
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ManhattanGMAT Staff


Posts: 6765
(1)
y = mx + b

substitute m = 3b --> y = 3bx + b
you can factor out b to give y = (3x + 1)b
this won't help us find b, so, insufficient.

(2)
graphically, we can draw a line through (-1/3, 0) with absolutely any slope we want. each of those lines will have a different y-intercept (with the exception of the one purely vertical line, which won't have a y-intercept at all).
insufficient.

(together)
substitute (-1/3, 0) into y = 3bx + b
0 = -b + b
0 = 0
so, nothing new here. this is a tautology (i.e., it's always true), so we haven't found out anything new.
so still insufficient.

ans (e)
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dtriffonov
 Post subject: Re: In the xy-plane, what is the y-intercept
 Post Posted: Mon Oct 05, 2009 9:42 am 
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Students


Posts: 4
thanks Ron.
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RonPurewal
 Post subject: Re: In the xy-plane, what is the y-intercept
 Post Posted: Sat Oct 24, 2009 7:13 am 
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ManhattanGMAT Staff


Posts: 6765
dtriffonov wrote:
thanks Ron.


sweetness
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