# X + |x| = y + |y| ?

1. Jul 31, 2011

x + |x| = y + |y| ??

1. The problem statement, all variables and given/known data
Draw the graph of x + |x| = y + |y|

3. The attempt at a solution
x + |x| = y + |y|
2x = y + |y| for x $\geq$ 0
0 = y + |y| for x < 0

2x = y + |y|
2x = 2y which is x = y for y $\geq$ 0
2x = 0 for y < 0

0 = y + |y|
0 = 2y for y $\geq$ 0
0 = 0 for y < 0

The answer is the graph y = x for x > 0 which i can find in my work but it is also the entire quadrant formed by x < 0 and y < 0. That quadrant i cant find in my work. Who knows how this quadrant is found?

greetz
Ivar

Last edited: Jul 31, 2011
2. Jul 31, 2011

### Quinzio

Re: x + |x| = y + |y| ??

Basically , $\forall(x,y): x<0, y<0$ satisfy the equation giving $0=0$

3. Jul 31, 2011

### ArcanaNoir

Re: x + |x| = y + |y| ??

x>0 and y>0
x>0 and y<0

You did not specifically address the two separate cases when x<0.
if x<0 and y>0? I know, I'm just saying make sure you've thought about it....
and x<0 and y<0?
And then of course, what about y is 0 or x is 0?

4. Jul 31, 2011

Re: x + |x| = y + |y| ??

I have added the missing cases that indeed were missing. Thank you.

In reply quinzo's comment I indeed understand that for each negative value for x and y results in 0 = 0 so the entire quadrant is a valid combination of x and y.

Still I'm unsure having proved that the entire quadrant is consists of possible solutions. Though i'm Not questioning they are. Have i Proved it with the added cases?

5. Aug 1, 2011