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Coordinate Geometry

This topic has 2 expert replies and 2 member replies
gmattesttaker2 Legendary Member Default Avatar
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Coordinate Geometry

Post Sat Aug 18, 2012 11:44 pm
Hello,

I was wondering if you can please help with the explanation here. This is from OG 13 P. 318

74) In the xy-coordinate plane, is point R equidistant from points (-3,-3) and (1,-3)?

1) The x-coordinate of point R is -1
2) Point R lies on the line y = -3

OA: A

Thanks a lot.

Best Regards,
Sri

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Stuart Kovinsky GMAT Instructor
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Post Mon Aug 20, 2012 8:56 pm
pemdas wrote:
Stuart Kovinsky wrote:
There are no bonus points on the GMAT for super-complicated math, so avoid it whenever possible!
we got lucky that two points of the line segment were given with the same y-coordinate, i.e. parallel to x-abscess. What if we have different points?
We didn't get lucky - the question was designed to reward people who saw the quick solution.

Most GMAT math questions are designed with 4 levels of reward:

1) negative reward, for people who spend time on it and get it wrong;
2) 0 reward, for people who recognize that they don't know how to solve and get it wrong, but don't waste time on the question;
3) positive reward, for people who take traditional approaches to questions - they get it right, but they spend a lot of time doing so; and
4) super positive reward, for people who find the most efficient solution to the problem - they get it right in a lot less time, leaving more time for other questions.

Never forget - while the GMAT tests math, it's not a math test; the primary skills tested on the GMAT are problem analysis and strategic thinking.

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pemdas Legendary Member Default Avatar
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Post Mon Aug 20, 2012 2:13 pm
Stuart Kovinsky wrote:
There are no bonus points on the GMAT for super-complicated math, so avoid it whenever possible!
we got lucky that two points of the line segment were given with the same y-coordinate, i.e. parallel to x-abscess. What if we have different points?

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pemdas Legendary Member Default Avatar
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Post Sun Aug 19, 2012 3:09 am
gmattesttaker2 wrote:
Hello,

I was wondering if you can please help with the explanation here. This is from OG 13 P. 318

74) In the xy-coordinate plane, is point R equidistant from points (-3,-3) and (1,-3)?

1) The x-coordinate of point R is -1
2) Point R lies on the line y = -3

OA: A
Thanks a lot.

Best Regards,
Sri
we need to use a distance formula here for point R (x,y): (x-(-3))^2+(y-(-3))^2=(x-1)^2+(y-(-3))^2 or (x+3)^2+(y+3)^2=(x-1)^2+(y+3)^2 or as evident from the left-hand side and right-hand side we have the same (y+3)^2, therefore, (x+3)^2=(x-1)^2 <-> 8x=-8 and x=-1
Now, if we encounter in any statement x=-1 we should get sufficient information about our point R with coordinates (x,y) as y is not considered in our distance relationship at all.
st(1) x=-1 ==> Sufficient
st(2) y=-3 but we don't know about x-coordinate, Not Sufficient

answer a

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Stuart Kovinsky GMAT Instructor
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Post Sun Aug 19, 2012 9:04 pm
gmattesttaker2 wrote:
Hello,

I was wondering if you can please help with the explanation here. This is from OG 13 P. 318

74) In the xy-coordinate plane, is point R equidistant from points (-3,-3) and (1,-3)?

1) The x-coordinate of point R is -1
2) Point R lies on the line y = -3

OA: A
Hi!

This question should take about 15 seconds if you actually draw out the x-y plane. There are no bonus points on the GMAT for super-complicated math, so avoid it whenever possible!

If you plot the 2 points, you can see that, since they have the same y-coordinate (-3), one point that's halfway between them is (-1, -3) (the difference in x-coordinates is 4, so if you travel 2 units from either point you'll end up in the middle).

Further, if you draw a vertical line through x=-1, you can see that every point on that line is equidistant from the two points given.

Accordingly, if we know that point R has an x-coordinate of -1, the answer will be YES; if point R has any other x-coordinate, the answer will be NO.

1) gives us the x-coordinate... sufficient!
2) doesn't give us the x-coordinate... insufficient!

(1) is suff, (2) isn't... choose A!

Moral of the story: on geometry word problems, always draw out what you know.

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