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In the xy-plane, what is the y-intercept of line l?

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In the xy-plane, what is the y-intercept of line l? [#permalink] New post 28 Feb 2010, 20:51
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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
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Re: GMATPrep: y-intercept [#permalink] New post 01 Mar 2010, 01:42
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testprep2010 wrote:
Source: GMATPrep

Please solve this question (with explanation):

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

[Reveal] Spoiler:
Ans E


Equation of a line in point intercept form is y=mx+b, where: m is the slope of the line; b is the y-intercept of the line (the value of y for x=0); x is the independent variable of the function y.

The question is b=?

(1) The slope of line l is 3 times its y-intercept --> m=3b. Not sufficient to calculate b.

(2) The x-intercept of line l is -1/3 --> x-intercept is the value of x for y=0 --> 0=-\frac{1}{3}m+b --> m=3b. Same info as above. Not sufficient.

(1)+(2) Nothing new. Not sufficient.

For more on this issue please check Coordinate Geometry chapter of Math Book (link in my signature).

Hope it helps.
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Re: GMAT PREP DS [#permalink] New post 23 Aug 2010, 19:47
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uzzy12 wrote:
In the xy-plane, what is the y-intercept of line l?

(1) The slope of line l is 3 itmes the y-intercept
(2) The x-intercept of line l is -1/3


My attempt:

Line equation - y = mx +c

We need the value of c which the y-intercept of the line.

Statement 1: Slope is 3 times the y intercept.

Hence dy/dx = 3c. or m = 3c or c = m/3. Without m we cannot identify the value of c. Insufficient.

Statement 2: X intercept of the line is -1/3.

X intercept => 0 = m*x+c
0 = m (-1/3) + c
c = (1/3) * m. Without m, we cannot determine c. Insufficient.

Combining both (1) and (2), we know m = 3c and c = (1/3)m and both are the same.

No way to identify c. Hence insufficient. Answer E.
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Re: What is the y-intercept of the line [#permalink] New post 18 Sep 2010, 15:26
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To find: y-intercept of the line. When the line intercepts the y axis, x=0. Hence the original equation y=mx+c becomes y=c. Question asks for the value of c.
Statement 1: Slope of the line is 3 times the y intercept. Hence m = 3c. Insufficient.

Statement 2: The x intercept is -1/3. x intercept means y is zero. Hence the original line equation becomes
0=m(-1/3) +c

m = 3c. Same as the first equation. Insufficient.

Combining both yields nothing. Hence the answer is E.
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Re: What is the y-intercept of the line [#permalink] New post 18 Sep 2010, 16:09
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y=mx+c
m --> Slope
c --> Y-int

This may be re-written as :
\frac{x}{-c/m} + \frac{y}{c} = 1

We know that in terms of intercept the equation of the line is :
\frac{x}{X-Int} + \frac{y}{Y-Int} = 1

So X-Int = -c/m, Y-Int = c

(1)m=3c ... Not enough to say what the value of c is

(2)-c/m = -1/3 .... Hence m=3c. We know this is not sufficient

(1)+(2) : Same as (1) or (2)

Hence, answer is {e} ...neither statement enough
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Re: Coordinate geometry [#permalink] New post 08 Jan 2012, 03:11
Let the eqn of line be y=mx+c
To get y-intercept x=0 i.e. y=c ?

1) m = 3c Insuff.

2) 0 = - m/3 + c or c= m/3 Insuff. (Same as stmt 1)

Hence E
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Re: Coordinate geometry [#permalink] New post 08 Jan 2012, 18:57
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Rephrase: y = mx+c, c=?

1. m=3c. y=3cx+c, insuff
2. 0=-1/3m+c, c= m/3. insuff.

Both equations unsolvable. insuff. E
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Re: Coordinate geometry [#permalink] New post 11 Jan 2012, 05:04
Thanks Guys.......
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Re: In the xy-plane, what is the y-intercept of line l? [#permalink] New post 09 Dec 2012, 18:32
I approached this problem as:
y = mx + c.....equation of line... (1)

to find C

1) m = 3c.... (2)

substitute (2) in (1)

y = 3cx + c
y = c(3x +1)...... (3)

2) X intercept = -1/3

2 implies the line passes through (-1/3,0)
doesnt give enough info for M

Combined
substitute point (-1/3,0) in (3)

0 = c(3 * -1/3 +1)
0 = c (-1+1 )
c = 0 ?

what did i do wrong ?
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Re: In the xy-plane, what is the y-intercept of line l? [#permalink] New post 10 Dec 2012, 01:41
uroopesh wrote:
I approached this problem as:
y = mx + c.....equation of line... (1)

to find C

1) m = 3c.... (2)

substitute (2) in (1)

y = 3cx + c
y = c(3x +1)...... (3)

2) X intercept = -1/3

2 implies the line passes through (-1/3,0)
doesnt give enough info for M

Combined
substitute point (-1/3,0) in (3)

0 = c(3 * -1/3 +1)
0 = c (-1+1 )
c = 0 ?

what did i do wrong ?


From 0=c(-1+1) --> 0=c*0, which holds true for ANY value of c not only for c=0. Also, notice that if c=0, then m=0, thus our line is y=0 which contradicts the statements.

Hope it's clear.
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RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory

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PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. NEW!!!

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Re: In the xy-plane, what is the y-intercept of line l?   [#permalink] 10 Dec 2012, 01:41
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