## Hard graph !

I know the answer is (E) , but HOW ?
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 Recognitions: Homework Help 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.
 Sorry,I'm not sure that the correct answer is (E) :(

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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
 Recognitions: Gold Member Science Advisor Staff Emeritus 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.
 I didn't understand ! :( Can you explain it more simply ?
 Recognitions: Homework Help Can you follow the procedure I outlined for you in post #2?
 Since the line passes throught the origin, and the slope builds an angle between 45 and 90 degrees, I can say that: $$y - x = a ( y + x), \ a > 1$$ Solve this for y. What do you get?

 Quote by Dickfore Since the line passes throught the origin, and the slope builds an angle between 45 and 90 degrees, I can say that: $$y - x = a ( y + x), \ a > 1$$ Solve this for y. What do you get?

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

so .. ?

Mentor
 Quote by KKK_123 y(1-a)= x(1+a) so .. ?
Solve for y.

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

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 Quote by KKK_123 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.
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