Algebraic Manipulation (Problem 1)

Algebra Level pending

Let x x be a positive integer such that the number ( x + 12 ) 5 \sqrt{(x+12)^5} satisfies the following conditions:

  • It is a perfect 4th power, but not a perfect 16th power, and

  • The sum of its digits is a prime number less than 10.

Submit your answer as this number.


The answer is 1024.

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1 solution

Mahdi Raza
Jun 3, 2020

Conditions important:

  • Power of 4 4 is only important because it includes the power of 2 2 and fractional powers of 16 16
  • Digit sum is a prime 10 \leq 10

Integers that satisfy all three conditions are all powers of four: 4 , 16 , 64 , 256 , 1024 4, 16, 64, 256, 1024 \ldots . Now, we can calculate whether x is an integer for these values

( x + 12 ) 5 = 4 x Z ( x + 12 ) 5 = 16 x Z ( x + 12 ) 5 = 64 x Z ( x + 12 ) 5 = 256 x Z ( x + 12 ) 5 = 1024 x = 4 \begin{aligned} \sqrt{(x+12)^{5}} = 4 &\implies x \ne \Z \\ \sqrt{(x+12)^{5}} = 16 &\implies x \ne \Z \\ \sqrt{(x+12)^{5}} = 64 &\implies x \ne \Z \\ \sqrt{(x+12)^{5}} = 256 &\implies x \ne \Z \\ \color{#20A900}{\sqrt{(x+12)^{5}} = 1024} & \color{#20A900} \implies x = 4 \end{aligned}

Since we are asked the value of ( x + 12 ) 5 \sqrt{(x+12)^{5}} for x = 4 x = 4 . We get the answer as 1024 \boxed{\color{#20A900}{1024}}

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