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If x + |x| + y = 7 and x + |y| - y =6 , then x + y =

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If x + |x| + y = 7 and x + |y| - y =6 , then x + y = [#permalink] New post 13 Jun 2010, 04:41
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If x + |x| + y = 7 and x + |y| - y =6 , then x + y =

A. 3
B. 4
C. 5
D. 6
E. 9
[Reveal] Spoiler: OA

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Last edited by Bunuel on 13 Aug 2012, 23:31, edited 1 time in total.
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Re: Interesting Absolute value Problem [#permalink] New post 13 Jun 2010, 05:42
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I did it by brute force, considering all possibilities. But I'm sure someone can come up with a way to quickly identify the signs of x and y.

x + |x| + y = 7 ....1

x + |y| - y = 6 .....2

Consider x < 0
=> |x| = -x
Substitute in 1

x - x + y = 7
y = 7

Substitute in 2

x + 7 - 7 = 6
x = 6

But this violates x < 0 So our assumption was incorrect.

Consider x > 0 and y > 0
=> |x| = x and |y| = y

Substitute in 2

x + y - y = 6
x = 6

Substitute in 1

6 + 6 + y = 7
y = -5

This violates y > 0 so our assumption was incorrect.

Consider x > 0 and y < 0
=> |x| = x and |y| = -y

Substituting in 1 and 2:

2x + y = 7 .....3
x - 2y = 6 .....4

Solving we get x = 4 and y = -1
x + y = 3

Pick A.
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Re: Interesting Absolute value Problem [#permalink] New post 13 Jun 2010, 06:01
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Expert's post
Hussain15 wrote:
If x + |x| + y = 7 and x + |y| - y =6 , then x + y =

A. 3
B. 4
C. 5
D. 6
E. 9


If x\leq{0}, then x + |x| + y = 7 becomes: x-x+y=7 --> y=7>0, but then x + |y| - y =6 becomes: x+y-y=6 --> x=6>0, which contradicts the initial assumption x\leq{0}. So x can not be \leq{0} --> hence x>0.

Similarly if y\geq{0}, then x + |y| - y =6 becomes: x+y-y=6 --> x=6>0, but then x + |x| + y = 7 becomes: x+x+y=12+y=7 --> y=-5<0, which contradicts the initial assumption y\geq{0}. So y can not be \geq{0} --> hence y<0.

So x>0 and y<0:
x+|x|+y=7 becomes: x+x+y=7 --> 2x+y=7;
x+|y|-y=6 becomes: x-y-y=6 --> x-2y=6.

Solving: x=4 and y=-1 --> x+y=3.

Answer: A.

I feel there is an easier way, but world cup makes it harder to concentrate.
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Re: Interesting Absolute value Problem [#permalink] New post 18 Jun 2010, 06:23
Bunuel wrote:
Hussain15 wrote:
If x + |x| + y = 7 and x + |y| - y =6 , then x + y =

A. 3
B. 4
C. 5
D. 6
E. 9


If x\leq{0}, then x + |x| + y = 7 becomes: x-x+y=7 --> y=7>0, but then x + |y| - y =6 becomes: x+y-y=6 --> x=6>0, which contradicts the initial assumption x\leq{0}. So x can not be \leq{0} --> hence x>0.

Similarly if y\geq{0}, then x + |y| - y =6 becomes: x+y-y=6 --> x=6>0, but then x + |x| + y = 7 becomes: x+x+y=12+y=7 --> y=-5<0, which contradicts the initial assumption y\geq{0}. So y can not be \geq{0} --> hence y<0.

So x>0 and y<0:
x+|x|+y=7 becomes: x-x+y=7 --> 2x+y=7;
x+|y|-y=6 becomes: x-y-y=6 --> x-2y=6.

Solving: x=4 and y=-1 --> x+y=3.

Answer: A.

I feel there is an easier way, but world cup makes it harder to concentrate.



Why when y<0 do we get -2y?
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Re: Interesting Absolute value Problem [#permalink] New post 18 Jun 2010, 06:28
Expert's post
GMATBLACKBELT720 wrote:
Bunuel wrote:
Hussain15 wrote:
If x + |x| + y = 7 and x + |y| - y =6 , then x + y =

A. 3
B. 4
C. 5
D. 6
E. 9


If x\leq{0}, then x + |x| + y = 7 becomes: x-x+y=7 --> y=7>0, but then x + |y| - y =6 becomes: x+y-y=6 --> x=6>0, which contradicts the initial assumption x\leq{0}. So x can not be \leq{0} --> hence x>0.

Similarly if y\geq{0}, then x + |y| - y =6 becomes: x+y-y=6 --> x=6>0, but then x + |x| + y = 7 becomes: x+x+y=12+y=7 --> y=-5<0, which contradicts the initial assumption y\geq{0}. So y can not be \geq{0} --> hence y<0.

So x>0 and y<0:
x+|x|+y=7 becomes: x-x+y=7 --> 2x+y=7;
x+|y|-y=6 becomes: x-y-y=6 --> x-2y=6.

Solving: x=4 and y=-1 --> x+y=3.

Answer: A.

I feel there is an easier way, but world cup makes it harder to concentrate.



Why when y<0 do we get -2y?


When y<0, then |y|=-y and x+|y|-y=6 becomes: x-y-y=6 --> x-2y=6.
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Re: Interesting Absolute value Problem [#permalink] New post 18 Jun 2010, 07:17
^
Wow... confusing as heck. Essentially saying that (hypothetical number here) |-3| = -(-3), thats fine. But I kept thinking you would apply this to -y and essentially make it +y since and make them both +y...
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Re: Interesting Absolute value Problem [#permalink] New post 14 Jun 2011, 02:25
for x>0 and x < 0
2x+ y = 7 and y = 7

for y>0 and y<0
x=6 and x-2y=6

for x,y<0 solution exists.
solving x=4 and y = -1

3
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Re: If x + |x| + y = 7 and x + |y| - y =6 , then x + y = [#permalink] New post 02 Jul 2013, 05:38
If x + |x| + y = 7 and x + |y| - y =6 , then x + y =

can be done in 2.3 mins :

there are 4 cases to be tested :
1) x is -ve and y is -ve
substituting in the equation , we get x-x+y=7 and x-y-y=6 solve for x and y we get x=20 and y=7 , so x+y=27 REJECT

2)x is +ve and y is +ve

substitute in the equation, we ger x+x+y=7 and x+y-y=6 solve for x and y we get x=6 and y=-5 ,therefore x+y=1 not on list so REJECT

3) x is -ve and y is +ve

substitute , we get x-x=y=7 and x+y-y=6 solve fo x and y we get x=6 and y=7, x+y=13 not on list so REJECT

4) x is +ve and y is -ve

substitute , we get x+x=y=7 and x-y-y=6 solve for x and y , we get x=4 and y= -1 ,x+y=3 , ANSWER CHOICE
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Re: If x + |x| + y = 7 and x + |y| - y =6 , then x + y = [#permalink] New post 02 Jul 2013, 10:22
I solved in 1.36 mins :)
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Re: Interesting Absolute value Problem [#permalink] New post 05 Jul 2013, 10:19
Bunuel wrote:
GMATBLACKBELT720 wrote:
Bunuel wrote:

If x\leq{0}, then x + |x| + y = 7 becomes: x-x+y=7 --> y=7>0, but then x + |y| - y =6 becomes: x+y-y=6 --> x=6>0, which contradicts the initial assumption x\leq{0}. So x can not be \leq{0} --> hence x>0.

Similarly if y\geq{0}, then x + |y| - y =6 becomes: x+y-y=6 --> x=6>0, but then x + |x| + y = 7 becomes: x+x+y=12+y=7 --> y=-5<0, which contradicts the initial assumption y\geq{0}. So y can not be \geq{0} --> hence y<0.

So x>0 and y<0:
x+|x|+y=7 becomes: x-x+y=7 --> 2x+y=7;
x+|y|-y=6 becomes: x-y-y=6 --> x-2y=6.

Solving: x=4 and y=-1 --> x+y=3.

Answer: A.

I feel there is an easier way, but world cup makes it harder to concentrate.



Why when y<0 do we get -2y?


When y<0, then |y|=-y and x+|y|-y=6 becomes: x-y-y=6 --> x-2y=6.


Sorry, still confusing me.
I understand the first y i.e.
|y| = -y
But y < 0,
so wouldn't x + |y| - y = x - y - (-y), which would make it just x?
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Re: Interesting Absolute value Problem [#permalink] New post 05 Jul 2013, 10:30
Expert's post
jjack0310 wrote:
Bunuel wrote:
GMATBLACKBELT720 wrote:

Why when y<0 do we get -2y?


When y<0, then |y|=-y and x+|y|-y=6 becomes: x-y-y=6 --> x-2y=6.


Sorry, still confusing me.
I understand the first y i.e.
|y| = -y
But y < 0,
so wouldn't x + |y| - y = x - y - (-y), which would make it just x?


When y<0, then |y|=-y: correct. But y must stay as it is.

Consider this, suppose we have only x-y=6, and I tell you that y is negative would you rewrite the equation as x-(-y)=6?

Hope it's clear.
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Re: If x + |x| + y = 7 and x + |y| - y =6 , then x + y = [#permalink] New post 09 Jul 2013, 15:24
If x + |x| + y = 7 and x + |y| - y =6 , then x + y =

x>0, y>0

x + (x) + y = 7
2x+y=7

x + |y| - y = 6
x + y - y = 6
x=6

2x+y=7
2(6)+y=7
y=-5

INVALID as 6>0 but -5 is not > 0

x>0, y<0

x + (x) + y = 7
2x+y=7

x + |y| - y = 6
x + (-y) - y = 6
x - 2y = 6
x = 6+2y

2(6+2y) + y = 7
12+4y+y=7
12+5y=7
5y+-5
y=-1

2x+y=7
2x + (-1) = 7
2x - 1 = 7
2x=8
x=4

VALID as 4>0 and -1<0

therefore;

x+y = (4)+(-1) = 3

(A)
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Re: If x + |x| + y = 7 and x + |y| - y =6 , then x + y = [#permalink] New post 03 Sep 2014, 10:18
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Re: If x + |x| + y = 7 and x + |y| - y =6 , then x + y =   [#permalink] 03 Sep 2014, 10:18
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