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# If m and n are integers and x>0, what is value of x^m + x^n?

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Intern
Joined: 21 Sep 2013
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If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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20 Jun 2014, 05:23
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If m and n are integers and x > 0, what is the value of $$x^m + x^n$$?

(1) $$x^m = 81$$

(2) $$1 + x^{n-m} = 10$$
[Reveal] Spoiler: OA

Kudos [?]: 7 [0], given: 54

Math Expert
Joined: 02 Sep 2009
Posts: 41892

Kudos [?]: 129046 [0], given: 12187

Re: If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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20 Jun 2014, 05:36
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If m and n are integers and x > 0, what is the value of $$x^m + x^n$$?

(1) $$x^m = 81$$. No info about n. Not sufficient.

(2) $$1 + x^{n-m} = 10$$ --> $$x^{n-m} = 9$$ --> $$\frac{x^n}{x^m}=9$$ --> $$x^n =9x^m$$ --> $$x^m + x^n=10x^m$$. Not sufficient.

(1)+(2) $$x^m + x^n=10x^m=10*81$$. Sufficient.

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Re: If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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20 Jun 2014, 05:52
newbornmuse wrote:
If m and n are integers and x > 0, what is the value of $$x^m + x^n$$?

(1) $$x^m = 81$$

(2) $$1 + x^{n-m} = 10$$

x^m + x^n= x^m(1+x^n-m)---------------------------A)

st1 x^m=81 insufficient as no information about n is provided

st2 (1 + x^n-m) =10

as seen from 'A' we still need the value of x^m to calculate the value of x^m+x^n hence insufficient

combining 1 and 2 we can substitute the value of 1 + x^n-m and x^m in 'A' to calculate the total value which is 81*10=810 hence C

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Manager
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Kudos [?]: 133 [0], given: 33

If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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09 Oct 2016, 13:47
I concluded that statement 2 was sufficient.

This is what I did.

X^m + X^n = 1 + (X^n)/(X^m)

Statement 2: 1 + (X^n)*(X^-m) = 10 ---> 1 + (X^n)/(X^m) = 10

Hence: X^m + X^n = 1 + (X^n)/(X^m) = 10 ---> Sufficient.

Can anyone please explain me what am I doing wrong?

Thank you.

Kudos [?]: 133 [0], given: 33

Math Expert
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Re: If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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10 Oct 2016, 00:04
EBITDA wrote:
I concluded that statement 2 was sufficient.

This is what I did.

X^m + X^n = 1 + (X^n)/(X^m)

Statement 2: 1 + (X^n)*(X^-m) = 10 ---> 1 + (X^n)/(X^m) = 10

Hence: X^m + X^n = 1 + (X^n)/(X^m) = 10 ---> Sufficient.

Can anyone please explain me what am I doing wrong?

Thank you.

How did you get the highlighted part?
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Kudos [?]: 129046 [0], given: 12187

Manager
Joined: 24 May 2016
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Kudos [?]: 133 [0], given: 33

If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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10 Oct 2016, 01:02
The question asks us for the value of: X^m + X^n

If we divide this expression by X^m, we get: 1 + (X^n)/(X^m)

Per statement 2: 1 + (X^n)/(X^m) = 10.

Hence, statement 2 allows us to know what is the value of the initial expression.

Where am I making the error?

Kudos [?]: 133 [0], given: 33

Math Expert
Joined: 02 Sep 2009
Posts: 41892

Kudos [?]: 129046 [1], given: 12187

Re: If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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10 Oct 2016, 01:07
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Expert's post
EBITDA wrote:
The question asks us for the value of: X^m + X^n

If we divide this expression by X^m, we get: 1 + (X^n)/(X^m)

Per statement 2: 1 + (X^n)/(X^m) = 10.

Hence, statement 2 allows us to know what is the value of the initial expression.

Where am I making the error?

I see what you are doing there but it's wrong. We need the value of x^m + x^n. When you divide this by x^m, you get the value of (x^m + x^n)/x^m, not x^m + x^n.
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Re: If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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13 Oct 2016, 12:36
I choose C but I was wondering if my approach was correct?
Here is what I did :
1) insuff.
2) insuff.

C) Given 1+x^n−m=10 I rewrote as 1+ x^n/x^m = 10 then substitute x^m from 1)
I got 1+ x^n/81 = 10
81+x^n=810
x^n=729
So If I got x^n and x^m to answer the question.

But is it a correct way to arrive at solution?
thank you

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Intern
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If m and n are integers and x>0, what is value of x^m + x^n? [#permalink]

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05 Jun 2017, 15:52
This is how GMAT prep solved it and it makes sense:

Statement 1
X^m = 81 insufficient as there we need two equations for two variables (n & m), therefore insufficient

Statement 2
1 + x^n - m = 10
we can try to solve it and see what gives us
so... x^n-m = 10 -1
we get ...
x^n-m = 9
we can then separate both n-m by doing the reciprocals:
then we get x^n / x^m = 9. This is why statement 2 is not sufficient since we cannot find the value of "n". Therefore, not sufficient.

Now, we are left with statements 1 + 2 and see if the problem can be solved
Remember statement 1? x^m = 81

Remember your result from statement 2?

now you can replace x^m = 81 in your second statement where x^n / 81 = 9. Then we can take x^n = 9*81, which is 279.

Then we have as a result that x^n = 279 and x^m = 81. Now you only need to sum both.... 81 + 279 = 810. Therefore, the answer is "C" not because we got "810" but because the problem can be solved. This is what data sufficiency questions are asking you to determine...can the problem be solved? In this case only when both statements are combined the problem can be solved.

Anyone from Guatemala please say hello!

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If m and n are integers and x>0, what is value of x^m + x^n?   [#permalink] 05 Jun 2017, 15:52
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# If m and n are integers and x>0, what is value of x^m + x^n?

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