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tinki
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thanks . yet im actually wondering what is the formula used here that comes to direct equation. ???

The equation of line 2 is \frac{y - 0}{\frac{1}{2} - 0} = \frac{x - 1}{0 - 1} or 2y = 1 - x
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sorry, here is the equation of my interest :

The equation of line 2 is \(\frac{y - 0}{\frac{1}{2} - 0} = \frac{x - 1}{0 - 1}\) or \(2y = 1 - x\) .
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The formula for finding the slope of a line is (change in y)/(change in x)

In your case, two coordinates (1,0) and (0,1/2) where (x,y)

(0-1/2)/(1-0) = -1/2 which is the slope of the line. The common formula for a line is y = kx + m where k is the slope and m is the y-intercept.

The first coordinate (1,0) tells us that when y = 0 x = 1, which is the y-intercept or 'm' in the formula y = kx+m where m is the intercept and 'k' is the slope. Pluggin in the slope (-1/2) and the intercept (1) into the formula gives ut y = 1-(x/2)

y- intercept is at x=0 as i know. so y= 1/2 when x=0 Am i missing something?
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Mackieman
The formula for finding the slope of a line is (change in y)/(change in x)

In your case, two coordinates (1,0) and (0,1/2) where (x,y)

(0-1/2)/(1-0) = -1/2 which is the slope of the line. The common formula for a line is y = kx + m where k is the slope and m is the y-intercept.

The first coordinate (1,0) tells us that when y = 0 x = 1, which is the y-intercept or 'm' in the formula y = kx+m where m is the intercept and 'k' is the slope. Pluggin in the slope (-1/2) and the intercept (1) into the formula gives ut y = 1-(x/2)

y- intercept is at x=0 as i know. so y= 1/2 when x=0 Am i missing something?

Sorry, you are correct, I had a rough day yesterday ;-) I hope it makes sense now.
(edited my post above)
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For every point \((a, b)\) lying on line 1, point \((b, -a)\) lies on line 2. If the equation of line 1 is \(y = 2x + 1\) , what is the equation of line 2 ?

(C) 2008 GMAT Club - [t]m18#24[/t]

* \(y = \frac{1}{2} + \frac{x}{2}\)
* \(2y = 1 - x\)
* \(\frac{x + y}{2} = -1\)
* \(y = \frac{x}{2} - 1\)
* \(x = 2y + 1\)

Find two points on line 2 and use their coordinates to build the line's equation. Points \((0, 1)\) and \((-\frac{1}{2}, 0)\) on line 1 correspond to points \((1, 0)\) and \((0, \frac{1}{2})\) on line 2. The equation of line 2 is \(\frac{y - 0}{\frac{1}{2} - 0} = \frac{x - 1}{0 - 1}\) or \(2y = 1 - x\) .
The correct answer is B.


i cant understand what the formula used here for building equation line 2. Can somebody explain? what is the Formula ?
thank you

Check this: math-coordinate-geometry-87652.html (chapter "Lines in Coordinate Geometry").
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I saw the link. VERY IMPRESSIVE!!!! + kudo from me

GREAT JOB !!!
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tinki
For every point \((a, b)\) lying on line 1, point \((b, -a)\) lies on line 2. If the equation of line 1 is \(y = 2x + 1\) , what is the equation of line 2 ?

(C) 2008 GMAT Club - [t]m18#24[/t]

* \(y = \frac{1}{2} + \frac{x}{2}\)
* \(2y = 1 - x\)
* \(\frac{x + y}{2} = -1\)
* \(y = \frac{x}{2} - 1\)
* \(x = 2y + 1\)

Find two points on line 2 and use their coordinates to build the line's equation. Points \((0, 1)\) and \((-\frac{1}{2}, 0)\) on line 1 correspond to points \((1, 0)\) and \((0, \frac{1}{2})\) on line 2. The equation of line 2 is \(\frac{y - 0}{\frac{1}{2} - 0} = \frac{x - 1}{0 - 1}\) or \(2y = 1 - x\) .
The correct answer is B.


i cant understand what the formula used here for building equation line 2. Can somebody explain? what is the Formula ?
thank you

Check this: math-coordinate-geometry-87652.html (chapter "Lines in Coordinate Geometry").


bunuel/Karishma

This is how I solved and it worked but I don't know why it worked :D Please enligthen me about why it worked...

I plugged in (a,b) in line 1 equeation to get b=2a+1..

then I started plugging in (b,-a) in the answer choices.. the 2nd answer choice resulted in b=2a+1.. and hence I marked B..
I am not sure why this worked.. Please help. .
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I plugged in (a,b) in line 1 equeation to get b=2a+1..

then I started plugging in (b,-a) in the answer choices.. the 2nd answer choice resulted in b=2a+1.. and hence I marked B..
I am not sure why this worked.. Please help. .

Responding to a pm:

a and b stand for two numbers which define a co-ordinate on the plane. When you say that (a, b) lies on y = 2x+1, it means the relation between a and b is b = 2a + 1. e.g. if a = 0, b = 1; if a = 1, b = 3... At the end of the day, a line is nothing but a depiction of how one variable changes with another. A line just shows you the relation between 2 variables.

If (b, -a) lies on a line 2y = 1-x, this is just a different way of expressing the same relation between the two numbers a and b.
a and b are the same set of numbers (i.e. if a = 0, b = 1; if a = 1, b = 3...)
So after manipulating the equation a little, you are bound to get b = 2a + 1 only.

As you figured out, the approach is a little un-intuitive. When I looked at the problem, I actually solved it exactly the same way except that I took numbers rather than a and b.

I said, if (a, b) lies on y = 2x + 1, if a = 1, b = 3.
So (3, -1) must lie on the new equation of the line. When I put (3, -1) in the options, I see that only (B) satisfies.
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it took me a while to get this

so what i did was create a value for x then sub it into line 1 to find out what y is. now that you have (a,b) create the point (b,-a).

now what you do is sub is points of line 2 into various equations until you find one that makes sense.

 
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Correct answer B,
GMAT club questions give me a reality check like nothing else :P
I found points where line one intercept x and y axis i.e (-1/2,0) and (0,1)
as (a,b) corresponds to (-b,a)..line 2 meets x and y axis at (1,0) and (0,1/2)
using 2 points we find slope = 0-(1/2)/1-0 = -1/2
we can see y intercept is 1/2 (point where line 2 intersects y axis)
line 2=> y=(-1/2)x + 1/2
hope it helps
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