What a root

Algebra Level 4

How many non-negative real values of x x are there, such that

10000 x 5 4 \sqrt[4]{ 10000 - \sqrt[5]{x} }

is an integer?


The answer is 11.

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10 solutions

Santanu Banerjee
Sep 29, 2013

Let the fifth root of 'x' be 'a'. The number 10000-a should be positive and its fourth root should be

an integer. We know that fourth root of 10000 is 10. Thus the final result after computing both the

roots will lie between 0 to 10 (inclusive) as we can find some or the other value of a (that is some

value of x) which satisfies this. Thus answer is the number of whole numbers upto 10.

Answer 11

If x=1,then it will be the 4th root of 9999,which is not a n integer.Then how is this correct?

Abin Das - 7 years, 8 months ago

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If you want the value of the expression to be 1, then we have 10000 x 5 4 = 1 \sqrt[4]{10000 - \sqrt[5]{x} } = 1 , or that 10000 x 5 = 1 10000 - \sqrt[5]{x} = 1 , or that x 5 = 9999 \sqrt[5]{x} = 9999 or that x = 999 9 5 x = 9999^5 .

The point of this question wasn't to find all the possible values of x x , but rather to recognize that the expression was a continuous decreasing function of x x , and hence would achieve all the values in it's range exactly once. Hence, we only need to count the number of integers in the range.

Calvin Lin Staff - 7 years, 8 months ago

It is not correct because x = 1 is not a solution ... and as far as I think x wont be any integer at least not for all the 11 solutions

Santanu Banerjee - 7 years, 8 months ago
Mohith Manohara
Sep 29, 2013

Because the values of x can only be positive, and since we know that negative numbers can't have fourth roots, we can figure out that the number of numbers has to be all of the numbers that have fourth roots < 10000. Since the fourth root of 10000 is 10, it has to be all of the positive integers and zero going up to 10. Including zero, you have 11 numbers, so the answer is 11 \boxed{11} .

nice!

Jung Min Lee - 7 years, 8 months ago

nice!!!!

Pamela Hsu - 7 years, 6 months ago
Adrabi Abderrahim
Sep 30, 2013

we've :

1 0 4 x 5 4 = k \sqrt[4]{10^4 - \sqrt[5]{x}} = k where k k is integer, so can be rewritten as :

1 0 4 x 5 2 n = k \sqrt[2n]{10^4 - \sqrt[5]{x}} = k where k , n k, n is integers, because the root is even implies than k k is non-negative integer, also can be rewritten as 1 0 4 x 5 = k \sqrt{\sqrt{10^4 - \sqrt[5]{x}}} = k .

by add power 4 4 to both sides:

1 0 4 x 5 4 = k \sqrt[4]{10^4 - \sqrt[5]{x}} = k

become:

1 0 4 x 5 = k 4 10^4 - \sqrt[5]{x} = k^4

x 5 = 1 0 4 k 4 \sqrt[5]{x} = 10^4 - k^4

we deduce that k 10 k \leq 10 because x x must be non-negative real number.

by replacing x 5 \sqrt[5]{x} by 1 0 4 k 4 10^4 - k^4 , we've:

1 0 4 x 5 4 = k \sqrt[4]{10^4 - \sqrt[5]{x}} = k

become:

1 0 4 ( 1 0 4 k 4 ) 4 = k \sqrt[4]{10^4 - (10^4 - k^4)} = k

k 4 4 = k \sqrt[4]{k^4} = k

then number of positive number from k = 0 k = 0 to k = 10 k = 10 is 11 11

I don't get this

Jonathan Lowe - 7 years, 8 months ago

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all of this?

ADRABI Abderrahim - 7 years, 8 months ago

every thing is fine except the last conclusion.. how do u get that.. ?

Alok Mishra - 7 years, 8 months ago

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because k k is integer (in question) and "even root" is non-negative, so k k is non-negative and started from 0 0 (smaller one) also we've k 10 k \leq 10 , so it's k = 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 k = 0,1,2,3,4,5,6,7,8,9,10 is 11 11 element, or can be 10 0 + 1 10 - 0 + 1

ADRABI Abderrahim - 7 years, 8 months ago

k<= 10?? How can u say tht?? especially the "=" part?

Sayan Chakraborty - 7 years, 8 months ago
Carl Denton
Oct 1, 2013

( 10000 x 1 / 5 ) 1 / 4 (10000 - x^{1/5} )^{1/4} is equivalent to ( 1 0 4 y ) 1 / 4 (10^4 - y)^{1/4} , where y = x 1 / 5 y = x^{1/5} . Note that since x 1 / 5 x^{1/5} can take on any real value, it can simply be replaced by another variable, in this case y y . For the expression to give an integer, 1 0 4 y = a 4 10^4 - y = a^4 , for some integer value a a . Rearranging, we have 1 0 4 a 4 = y 10^4 - a^4 = y . Since y cannot be negative, a 10 a \leq 10 . a a can therefore take on any integer value from 0 to 10 inclusive, giving a total of 11 possible values for x 1 / 5 x^{1/5} .

nice :D

Leo Kudo - 7 years, 8 months ago
Daniel Ferreira
Sep 30, 2013

O menor valor possível que "x" pode assumir, de acordo com o enunciado, é zero; com isso:

10000 x 5 4 = 10000 0 4 = 10 \\ \sqrt[4]{10000 - \sqrt[5]{x}} = \\\\ \sqrt[4]{10000 - 0} = \\\\ \boxed{10}

E, o maior valor possível que a expressão pode assumir é 10!

Para concluir o problema não se faz necessário descobrir o maior valor que "x" assume, mas sim, o menor valor que a expressão...

Ora, se a expressão tem como valores os números inteiros, o menor valor da expressão é o próprio zero, uma vez que não poderá ser negativo, afinal trata-se de números inteiros e não complexos!

Logo,

10 0 + 1 = 11 10 - 0 + 1 = \\\\ \boxed{\boxed{11}}

You double boxed your answer

Yow Ka Shing - 7 years, 8 months ago

I will try to explain myself

Mayankk Bhagat
Jan 25, 2014

let t= (10000 -(x)^(1/5) ) ^(1/4)).................. T can only be an integer when t^4=0,1,16,81,.........10^4......i.e. a tally of 11 numbers and hence for 11 corresponding non negative values of X.

For the expression to be +tive integer, the max is 10. Thus they are only
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10........only 11.

Niranjan Khanderia - 7 years, 2 months ago
Abrar Nihar
Sep 30, 2013

As x x can be any non-negative real value and the outcome is an integer, it is clear that x x itself must be a non-negative integer...

We can substitute x x by the 5 5 th power of any non-negative integer such that y : = x 5 y := \sqrt[5]{x} can be any non-negative integer...

Hence, all we have to find is the number of all 4 4 th powers n n such that n 10000 n \leq 10000 ...

Since, 1 0 4 = 10000 10^4=10000 and x = 0 x = 0 is also a solution, there are 10 + 1 = 11 10+1=\fbox{11} non-negative real values satisfying the conditions in the question...!!!

Meike Rouwenhorst
Sep 30, 2013

The largest outcome is 10, then x has to be 0.

If 9 were an outcome, then x = ( 10000 9 4 ) 5 x = (10000 - 9^4)^5 , which is an non-negative real value. If -9 were an outcome, then x = ( 10000 ( 9 ) 4 ) 5 x = (10000 - (-9)^4)^5 which is the non-negative real value of x . There are 21 possible outcomes: -10, -9 ... 0, 1, 2 ... 10, so there are 11 non-negative real values of x .

Omar Pulido
Oct 5, 2013

Actually I thought this was pretty easy. No, I did not compute the actual values of the possible values of x, but I used my common sense.

What are the integer values of something to the fourth power that are less than or equal to ten-thousand?

0 4 = 0 0^4 = 0

1 4 = 1 1^4 =1

2 4 = 16 2^4=16

3 4 = 81 3^4=81

4 4 = 256 4^4=256

5 4 = 625 5^4=625

6 4 = 1296 6^4=1 296

7 4 = 2401 7^4= 2 401

8 4 = 4096 8^4=4 096

9 4 = 6561 9^4=6 561

1 0 4 = 10000 10^4=10 000

So we can stop at 1 0 4 10^4 , because and thing greater would mean that x would be negative, and anything less than zero would mean that the number would be imaginary.

So there are 11 \boxed{11} ways

it is a general solution "it wasn't written in the assumptions there is no range !!!!" example to make this equation integer = 9 x must be 4.8* 10^17 !!!!!!

Mohamed Abdalla - 7 years, 8 months ago

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Well we know the range is going from 0 to 10 000 The rang cannot be greater than 10 000, because that would mean that x would be negative, and the rang cannot be negative because the output has to be an integer.

Omar Pulido - 7 years, 8 months ago

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