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TestPrep Geometry

by moneyman » Mon Feb 25, 2008 7:42 am
In the xy plane, what is the y intercept of line l ?

(1)The slope of line l is 3 times the y intercept

(2)The x intercept of line l is -1/3

OA is E
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by camitava » Mon Feb 25, 2008 8:27 am
Moneyman,
slope = y-intercept/ x -intercept = y/x
now by stmt-1, y/x = 3y so x = 1/3
and by stmt-2, x = -1/3.
But we are getting any clue abt the value of y. so imo e.
Correct me If I am wrong


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by moneyman » Mon Feb 25, 2008 8:29 am
Can you pls explain how slope=y/x??
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by camitava » Mon Feb 25, 2008 8:31 am
If u say y-intercept = y and x-intercept = x, then slope = y/x. Is it clear to u now?
Correct me If I am wrong


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by [email protected] » Sun Jun 29, 2008 1:30 pm
Is it okay to look at this questions as follow...

y = mx + b

After statement one we know:

3m = y - b

After statement two we know:

0 = -1/3m +b

Even with both statements, we are left with three equations and four unknowns, not enough to solve??
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by Ian Stewart » Sun Jun 29, 2008 2:02 pm
[email protected] wrote:Is it okay to look at this questions as follow...

y = mx + b

After statement one we know:

3m = y - b

After statement two we know:

0 = -1/3m +b

Even with both statements, we are left with three equations and four unknowns, not enough to solve??
No, that's not correct. We want the y-intercept; in other words we want to find the point on the line when x = 0. So x is known in advance. Also, the y-coordinate we are looking for is b, so we only have two unknowns here: m and b. The problem is that the two statements give us the same equation, so while we appear at first to have two equations, two unknowns, we really only have one equation, two unknowns. You haven't translated the statements correctly into equations:

(1)The slope of line l is 3 times the y intercept

This tells you m = 3b

(2)The x intercept of line l is -1/3

This tells you that (-1/3, 0) is on the line, i.e. that 0 = m*(-1/3) + b. You can rearrange this, and it just says that m = 3b, exactly what we found from (1).
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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