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Hard graph !

by ABC_123
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ABC_123
#1
Mar7-12, 07:07 AM
P: 5




I know the answer is (E) , but HOW ?
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Mentallic
#2
Mar7-12, 07:28 AM
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Are you sure it's meant to be E) because I'm getting D)

If you were given a linear graph with the vertical axis being Y, and the horizontal being X, then you could say that Y=mX for some constant m, right?

You can similarly substitute each axis representation into this equation. The vertical is now y-x and the horizontal x+y so let Y=y-x and X=x+y and then rearrange the equation to make y the subject.
ABC_123
#3
Mar7-12, 07:34 AM
P: 5
Sorry,I'm not sure that the correct answer is (E) :(

Mentallic
#4
Mar7-12, 08:28 AM
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Hard graph !

No worries. On the other hand, I'm quite sure the answer is D, but it doesn't even matter, we should just let the math do the talking
HallsofIvy
#5
Mar7-12, 08:46 AM
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The first graph is a straight line passing through the origin so it must be of the form y- x= k(x+ y). That is the same as y- x= kx+ ky or y- ky= x+ kx. Factoring, (1-k)y= (1+k)x or y= (1+k)/(1-k) x. From the picture it appears that k is larger than 1 (the angle is greater than 45 degrees) so 1-k is negative. That, in turn, means that the fraction (1+k)/(1-k) is negative so the graph goes downward to the right. It also follows that k< -1.
ABC_123
#6
Mar7-12, 10:02 AM
P: 5
I didn't understand !
:(


Can you explain it more simply ?
Mentallic
#7
Mar7-12, 08:32 PM
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Can you follow the procedure I outlined for you in post #2?
Dickfore
#8
Mar7-12, 08:52 PM
P: 3,014
Since the line passes throught the origin, and the slope builds an angle between 45 and 90 degrees, I can say that:
[tex]
y - x = a ( y + x), \ a > 1
[/tex]

Solve this for y. What do you get?
ABC_123
#9
Mar8-12, 08:17 AM
P: 5
Quote Quote by Dickfore View Post
Since the line passes throught the origin, and the slope builds an angle between 45 and 90 degrees, I can say that:
[tex]
y - x = a ( y + x), \ a > 1
[/tex]

Solve this for y. What do you get?

y(1-a)= x(1+a)

so .. ?

SammyS
#10
Mar8-12, 09:31 AM
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Quote Quote by KKK_123 View Post
y(1-a)= x(1+a)

so .. ?

Solve for y.

Plug-in some positive numbers greater than 1, for a.

BTW: Mentallic is right about what's the correct answer .
NascentOxygen
#11
Mar10-12, 06:37 AM
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Quote Quote by KKK_123 View Post
Can you explain it more simply ?
That graph in the picture appears to have a slope of about 3, for each mm horizontally it rises about 3mm, roughly (and assuming the scales are the same).

Usually we would express that graph as being the plot: y = 3x

But on this particular graph, the vertical quantity is given as y-x
and the variable on the horizontal is given as y+x
So substitute these expressions for the y and x in the blue equation, respectively.

Finally, rearrange the result to get y on one side by itself.
Redbelly98
#12
Mar13-12, 06:33 PM
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