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For all non zero integers n, n*=(n+2)/n. What is the value

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Joined: 22 Jul 2013
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GMAT 1: 650 Q48 V31
For all non zero integers n, n*=(n+2)/n. What is the value  [#permalink]

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New post Updated on: 27 Oct 2013, 06:40
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A
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C
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64% (01:27) correct 36% (01:35) wrong based on 301 sessions

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For all non zero integers n, n*=(n+2)/n. What is the value of x ?

(1) x* = x.
(2) x* = -2-x.

Answer = C.

For statement 2 , the solution says
=>(x+2)/x= -2-x.
=> Multiply both sides by x.
=> (x+2) = -x(x+2)
=> x+2= -\(x^2\) - 2x
=>\(x^2\) + 3x + 2 = 0.
(x+1)(x+2) = 0
x = -1 . x = -2

My question is why cant we cancel out the (x+2) on LHS and RHS instead of multiplying it by x in step.
We will get x = -1.
So answer should be B instead of C.

Why cant we reduce the equation ? Shall we never do it in GMAT ?
Could anyone please explain where equations should be reduced and where they shouldn't be

Originally posted by NeetiGupta on 26 Oct 2013, 18:20.
Last edited by Bunuel on 27 Oct 2013, 06:40, edited 2 times in total.
Edited the question.
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Re: For all nonzero integers n,n*=(n+2)/n.What is the value of x  [#permalink]

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New post 26 Oct 2013, 18:51
1
NeetiGupta wrote:
Q. For all non zero integers n,n*=(n+2)/n. What is the value of x ?
1. x* = x.
2. x* = -2-x.

Answer = C.

For statement 2 , the solution says
=>(x+2)/x= -2-x.
=> Multiply both sides by x.
=> (x+2) = -x(x+2)
=> x+2= -\(x^2\) - 2x
=>\(x^2\) + 3x + 2 = 0.
(x+1)(x+2) = 0
x = -1 . x = -2

My question is why cant we cancel out the (x+2) on LHS and RHS instead of multiplying it by x in step.
We will get x = -1.
So answer should be B instead of C.

Why cant we reduce the equation ? Shall we never do it in GMAT ?
Could anyone please explain where equations should be reduced and where they shouldn't be

you can only cancel a factor if it is nonzero. In this case, if you cancel (x+2), you also skip (loose) the root (x+2=0).
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GMAT 1: 650 Q48 V31
Re: For all nonzero integers n,n*=(n+2)/n.What is the value of x  [#permalink]

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New post 26 Oct 2013, 19:19
tuanle wrote:
NeetiGupta wrote:
Q. For all non zero integers n,n*=(n+2)/n. What is the value of x ?
1. x* = x.
2. x* = -2-x.

Answer = C.

For statement 2 , the solution says
=>(x+2)/x= -2-x.
=> Multiply both sides by x.
=> (x+2) = -x(x+2)
=> x+2= -\(x^2\) - 2x
=>\(x^2\) + 3x + 2 = 0.
(x+1)(x+2) = 0
x = -1 . x = -2

My question is why cant we cancel out the (x+2) on LHS and RHS instead of multiplying it by x in step.
We will get x = -1.
So answer should be B instead of C.

Why cant we reduce the equation ? Shall we never do it in GMAT ?
Could anyone please explain where equations should be reduced and where they shouldn't be

you can only cancel a factor if it is nonzero. In this case, if you cancel (x+2), you also skip (loose) the root (x+2=0).



Can you please elaborate "you can only cancel a factor if it is nonzero"
Do you mean when we take x=-2. x=2 becomes 0 and hence we cannot cancel it?
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Re: For all non zero integers n, n*=(n+2)/n. What is the value  [#permalink]

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New post 27 Oct 2013, 06:50
2
1
NeetiGupta wrote:
For all non zero integers n, n*=(n+2)/n. What is the value of x ?

(1) x* = x.
(2) x* = -2-x.

Answer = C.

For statement 2 , the solution says
=>(x+2)/x= -2-x.
=> Multiply both sides by x.
=> (x+2) = -x(x+2)
=> x+2= -\(x^2\) - 2x
=>\(x^2\) + 3x + 2 = 0.
(x+1)(x+2) = 0
x = -1 . x = -2

My question is why cant we cancel out the (x+2) on LHS and RHS instead of multiplying it by x in step.
We will get x = -1.
So answer should be B instead of C.

Why cant we reduce the equation ? Shall we never do it in GMAT ?
Could anyone please explain where equations should be reduced and where they shouldn't be


For all non zero integers n, n*=(n+2)/n. What is the value of x ?

(1) x* = x --> \(\frac{x+2}{x}=x\) --> \(x^2-x-2=0\) --> \(x=-1\) or \(x=2.\) Not sufficient.

(2) x* = -2-x --> \(\frac{x+2}{x}=-2-x\) --> \(x+2=-x(x+2)\) --> \((x+2)(1+x)=0\) --> \(x=-1\) or \(x=-2.\) Not sufficient.

If you divide (reduce) \(x+2=-x(x+2)\) by x+2 you assume, with no ground for it, that x+2 does not equal to zero thus exclude a possible solution (notice that both x=-1 AND x=-2 satisfy the equation).

Never reduce equation by variable (or expression with variable), if you are not certain that variable (or expression with variable) doesn't equal to zero. We can not divide by zero.

(1)+(2) Common value of x from (1) and (2) is \(x=-1\). Sufficient.

Answer: C.

Similar question to practice: for-all-integers-n-n-n-n-1-what-is-the-value-of-x-155982.html

Hope this helps.
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Re: For all non zero integers n, n*=(n+2)/n. What is the value  [#permalink]

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New post 17 Dec 2015, 07:16
(1) and (2) are certainly insufficient because they have two solutions each. But how to use (1) and (2) together, simple:
(1)=(2)
x=-2-x
2x=-2
x=-1 (one solution) sufficient
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Re: For all non zero integers n, n*=(n+2)/n. What is the value  [#permalink]

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New post 19 Oct 2017, 02:25
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Re: For all non zero integers n, n*=(n+2)/n. What is the value &nbs [#permalink] 19 Oct 2017, 02:25
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