y-intercept

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Deepthi Subbu: Master | Next Rank: 500 Posts; Posts: 298; Joined: Tue Feb 16, 2010 1:09 amFollowed by: 1 member; Thanked: 2 times

by Deepthi Subbu » Sat Jan 08, 2011 10:00 pm

What are the information that we need to know if a question asks us to find the y-intercept of a line .

For example : We just have the slope or may be an x-coordinate . Is this data enough?

So I want to know the information that is sufficient to find the y intercept of a line.

Someone please help.

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Source: Beat The GMAT — Data Sufficiency | Sat Jan 08, 2011 10:00 pm

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sun Jan 09, 2011 12:40 am

Deepthi Subbu wrote:What are the information that we need to know if a question asks us to find the y-intercept of a line .

For example : We just have the slope or may be an x-coordinate . Is this data enough?

So I want to know the information that is sufficient to find the y intercept of a line.

Someone please help.

Consider the slope-intercept equation of a line, i.e. (y = mx + c)
Here c is the y-intercept and m is the slope of the line.

Now if we know

1. only the slope, we cannot determine c.
2. the co-ordinates of a single point on the line, we cannot determine c.

BUT, if we know

1. the slope and coordinates of a point on the line, we can determine c.
2. the co-ordinates of two points on the line, we can determine c.

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Deepthi Subbu: Master | Next Rank: 500 Posts; Posts: 298; Joined: Tue Feb 16, 2010 1:09 amFollowed by: 1 member; Thanked: 2 times

by Deepthi Subbu » Sun Jan 09, 2011 1:24 am

Anurag@Gurome wrote:
Deepthi Subbu wrote:What are the information that we need to know if a question asks us to find the y-intercept of a line .

For example : We just have the slope or may be an x-coordinate . Is this data enough?

So I want to know the information that is sufficient to find the y intercept of a line.

Someone please help.
Consider the slope-intercept equation of a line, i.e. (y = mx + c)
Here c is the y-intercept and m is the slope of the line.

Now if we know
1. only the slope, we cannot determine c.
2. the co-ordinates of a single point on the line, we cannot determine c.
BUT, if we know
1. the slope and coordinates of a point on the line, we can determine c.
2. the co-ordinates of two points on the line, we can determine c.

Thank you .

What if the question says the slope is 3 times the y intercept and the x- intercept is also given .

I understand from your explanation that the data provided above is insufficient . But can you explain further the reason?

For eg : Slope = 3c and x-intercept is -1/3

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sun Jan 09, 2011 1:57 am

Deepthi Subbu wrote:What if the question says the slope is 3 times the y intercept and the x- intercept is also given .

I understand from your explanation that the data provided above is insufficient . But can you explain further the reason?

For eg : Slope = 3c and x-intercept is -1/3

They are equivalent information.
One leads to another but doesn't give any new information.

x-intercept is the value of x, when y = 0, in the equation y = mx + c
Thus, x-intercept = -(c/m)
Now if slope, m = 3c => (c/m) = 1/3
Hence, x-intercept = -(1/3)

Which is also given.

Last edited by Anurag@Gurome on Mon Jan 10, 2011 8:29 am, edited 1 time in total.

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prachich1987: Legendary Member; Posts: 752; Joined: Sun Sep 12, 2010 2:47 amFollowed by: 10 members; Thanked: 20 times; GMAT Score:700

by prachich1987 » Mon Jan 10, 2011 8:10 am

Anurag@Gurome wrote:
Deepthi Subbu wrote:What if the question says the slope is 3 times the y intercept and the x- intercept is also given .

I understand from your explanation that the data provided above is insufficient . But can you explain further the reason?

For eg : Slope = 3c and x-intercept is -1/3
No, it is not insufficient.

x-intercept is the value of x, when y = 0, in the equation y = mx + c
Thus, x-intercept = -(c/m)
Now if slope, m = 3c => (c/m) = 1/3
Hence, x-intercept = -(1/3)

How can it be sufficient?
Can you advise the equation of such line?

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Mon Jan 10, 2011 8:31 am

prachich1987 wrote:How can it be sufficient?
Can you advise the equation of such line?

Sorry, that was a typo.
I meant to write "... it is not sufficient."
Modified the reply. Check it.

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