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# GMATPrep: coordinate plane

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Intern
Joined: 07 May 2006
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GMATPrep: coordinate plane [#permalink]  17 Jun 2006, 12:46
Could you please explain how do we get the right answer (marked in blue box)

Thanks
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Manager
Joined: 17 Dec 2005
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I got C

C1. product of the two x-intercepts is positive

two cases:

line k = b/m = +ve
line l = b/m = +ve

line k = b/m = -ve
line l = b/m = -ve

so m can be either +ve for both m or -ve or one and +ve for the other

INSUFFICIENT

C2. product of the y-intercept is -ve

2 cases

line k -> b = +ve,
line l -> b = -ve

line k -> b = -ve
line l-> b = +ve

tells you nothing about the respective slopers

combined:

if line k's b/m = +ve and b = +ve then m = -ve
then line l ->b/m = +ve and b = -ve then m = +ve

product is negative

if line k's b/m = -ve and b = -ve then m = +ve
then line l -> b/m = +ve and b = +ve then m= -ve

product is -ve.
Manager
Joined: 29 Sep 2005
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Mrmikec, could you please explain the basic equations involved here?

I guess I am having trouble understanding how you ascertain slope from x and y intercepts?

b/m = x intercept? where b is y intercept and m is slope?

Any help is much appreciated.
Thanks
Manager
Joined: 17 Dec 2005
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tapan22 wrote:
Mrmikec, could you please explain the basic equations involved here?

I guess I am having trouble understanding how you ascertain slope from x and y intercepts?

b/m = x intercept? where b is y intercept and m is slope?

Any help is much appreciated.
Thanks

right m is slope and b is the y-intercept

the x-intercept formula is -b/m

hope this helps.
VP
Joined: 02 Jun 2006
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Going with C...
Let line l intersect a point (x1, y1) other than (4,3) and k intersect (x2, y2) other than (4,3)

Stmt1: x1 * x2 = +ve
We could have :
x1= +ve => y1 => -ve or +ve
x2 =+ve => y2 => -ve or +ve

OR
x1 = -ve => y1 => +ve
x2 = -ve => y2 => +ve

Either case, prod of slopes in +ve/-ve
Not sufficient.

Stmt2: y1*y2 = -ve
Following the same logic above this is not sufficient.

Together:
Prod of slopes = (-y2/x2) x (-y1/x1)
= y2*y1/x2*x1 = -ve/+ve

Sufficient.
Therefore C
Senior Manager
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It would also be easier to take examples and scribble few lines to counter the argument .. (C) makes sense as it would always yield descending and ascending lines, resultingin product of the slopes -ve.
Director
Joined: 06 May 2006
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C.

Draw a rough sketch. A line that makes an acute angle with the x-axis has a positive slope, and one that makes an obtuse angle with the x-axis has a negative slope.
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