There are integers satisfying an integer?

Algebra Level 2

Find all integer x x such that the expression x 3 x + 2 x \dfrac{x-3\sqrt{x} + 2}{\sqrt{x}} is an integer.

Type your answer as the sum of all solutions.


The answer is 5.

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2 solutions

Mr. India
Apr 20, 2019

x 3 x + 2 x \frac{x-3\sqrt{x}+2}{\sqrt{x}} = x 3 + 2 x =\sqrt{x}-3+\frac{2}{\sqrt{x}}

For this to be an integer, x x must be a perfect square and x \sqrt{x} must divide 2 2

Factors of 2 : 1 , 2 , 1 , 2 2 : 1,2,-1,-2

So, x = 1 , 2 , 1 , 2 \sqrt{x}=1,2,-1,-2 or x = 1 , 4 x=1,4

Answer is sum of values of x x , that is, 5 \boxed{5}

Since x is an integer, x \sqrt{x} doesn't have any negative values. Hence you can ignore -1 and -2.

Tin Le - 2 years, 1 month ago
Alex Burgess
May 21, 2019

I think the answer should be 3 3 , with solutions 2 , 1 , 4 -2, 1, 4 . (But would probably be better if you specified "positive integer" instead.

It's an integer iff ( 1 + 2 x ) x (1 + \frac{2}{x}) \sqrt{x} is an integer. This occurs when, x \sqrt{x} in an integer and so is 2 x \frac{2}{\sqrt{x}} , e.g x = 1 , 4 x = 1, 4 . Or when ( 1 + 2 x ) = 0 (1 + \frac{2}{x}) = 0 . In this case x = 2 i \sqrt{x} = \sqrt{2} i and is an imaginary number.

Also note, x 3 x + 2 x = ( x 1 ) ( x 2 ) x = ( y 1 ) ( y 2 ) y \frac{x - 3\sqrt{x} + 2}{\sqrt{x}} = \frac{(\sqrt{x}-1)(\sqrt{x}-2)}{\sqrt{x}} = \frac{(y-1)(y-2)}{y} . Which is 0 0 when y = 1 , 2 y = 1, 2 .

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