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# If x^4+y^4=100, then the greatest possible value of x is

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Manager
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If x^4+y^4=100, then the greatest possible value of x is [#permalink]

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23 Aug 2008, 04:22
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63% (01:38) correct 37% (00:59) wrong based on 361 sessions

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If x^4+y^4=100, then the greatest possible value of x is between

A. 0 and 3
B. 3 and 6
C. 6 and 9
D. 9 and 12
E. 12 and 15

OPEN DISCUSSION OF THIS QUESTION IS HERE: if-x-4-y-4-100-then-the-greatest-possible-value-of-x-is-128171.html
[Reveal] Spoiler: OA

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kris

Last edited by Bunuel on 19 Jul 2013, 02:54, edited 2 times in total.
Renamed the topic, edited the question, added the OA and moved to PS forum.
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Re: Greatest Possible values of x? [#permalink]

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23 Aug 2008, 04:41
krishan wrote:
If x4 + y4 = 100 then the greatest possible value of x is between:

a) 0 and 3
b) 3 and 6
c) 6 and 9
d) 9 and 12
e) 12 and 15

x4 is x raised to the power of 4 (I could not figure out how to show it as an exponent)

Thanks

Greatest possible value of X when Y^4 is minimum.
The possible minimum value of Y^4 is 0
$$X^4=100 ---> X^2=10 --> X> 3 AND X<4$$
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Manager
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Re: Greatest Possible values of x? [#permalink]

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23 Aug 2008, 05:32
Thanks a lot Suresh.
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kris

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Re: Greatest Possible values of x? [#permalink]

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24 Aug 2008, 00:09
shailyam,
The ans should be B not A since values geater than 3 also satify the equation e.g. x = 3.1 can also be possible.
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Re: Greatest Possible values of x? [#permalink]

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19 Jul 2013, 02:00
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LoyalWater wrote:
shailyam,
The ans should be B not A since values geater than 3 also satify the equation e.g. x = 3.1 can also be possible.

Going by your logic, one can consider 3.1 as a possible value of x. Here we're not sure of the value of x and saying that the greatest possible value of x is between 3 and 6 would mean that x could be even more than 3.1, for example 4. Hence, answer is A
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Re: Greatest Possible values of x? [#permalink]

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19 Jul 2013, 02:54
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kshitij90 wrote:
LoyalWater wrote:
shailyam,
The ans should be B not A since values geater than 3 also satify the equation e.g. x = 3.1 can also be possible.

Going by your logic, one can consider 3.1 as a possible value of x. Here we're not sure of the value of x and saying that the greatest possible value of x is between 3 and 6 would mean that x could be even more than 3.1, for example 4. Hence, answer is A

That's not correct.

If x^4+y^4=100, then the greatest possible value of x is between

A. 0 and 3
B. 3 and 6
C. 6 and 9
D. 9 and 12
E. 12 and 15

General rule for such kind of problems:
to maximize one quantity, minimize the others;
to minimize one quantity, maximize the others.

So, to maximize $$x$$ we should minimize $$y^4$$. Least value of $$y^4$$ is zero. In this case $$x^4+0=100$$ --> $$x^4=100$$ --> $$x^2=10$$ --> $$x=\sqrt{10}\approx{3.2}$$, which is in the range (3,6).

Hope it's clear.

OPEN DISCUSSION OF THIS QUESTION IS HERE: if-x-4-y-4-100-then-the-greatest-possible-value-of-x-is-128171.html
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Re: Greatest Possible values of x?   [#permalink] 19 Jul 2013, 02:54
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