Siam's inequality

Algebra Level 3

What is the minimum integer value of x x that satisfies the equation

x x + 34 = 0 ? \lfloor{\sqrt{x}}\rfloor - \lfloor{\sqrt{x+34}}\rfloor=0 ?

This problem is posed by Siam H .

Details and assumptions

The function x : R Z \lfloor x \rfloor: \mathbb{R} \rightarrow \mathbb{Z} refers to the greatest integer smaller than or equal to x x . For example 2.3 = 2 \lfloor 2.3 \rfloor = 2 and 5 = 5 \lfloor -5 \rfloor = -5 .


The answer is 289.

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6 solutions

Vicky Bro
Aug 19, 2013

Since we have to find out the square roots of x x and ( x + 34 ) (x+34) , basically we want to find two consecutive numbers whose squares don't differ by less than 34 34 . Let y y and y + 1 y+1 be the consecutive numbers, so the difference of their squares is

( y + 1 ) 2 y 2 = 2 y + 1 (y+1)^2 - y^2 = 2y+1

Now, we want this equal to 34 34 , which yields y = 16.5 y = 16.5 . Now let x = y 2 x = y^2 . But y 2 y^2 is not an integer. The closest number to 16.5 16.5 whose square is an integer is 17 17 . So y = 17 y = 17 and hence x = y 2 = 289 x = y^2 = 289 .

ok good solution.

ronie omandam jr. - 7 years, 9 months ago

Y cant it be 6

incredible mind - 6 years, 7 months ago

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Assume 6 is an answer. The floor of the square root of 6 is 2, the floor of the square root of 6+34 or 40 is 6. Because 2 is not equal to 6, 6 is not an answer.

Kevin Tong - 3 years, 7 months ago

The difference of the squares actually have to be greater than 34 in this case, since x is an integer. Assume the difference of their squares is exactly 34 and x=n^2. If this were the case, the floor of the first square root is n, and the floor of the second square root is the floor of n+1, which, since it is an integer, is itself. Since n is not equal to n+1, that would not work. Of course, if x did not have to be an integer, your case would work.

Kevin Tong - 3 years, 7 months ago

I assumed that x could be negative:D

Jeff Giff - 1 year ago
René Christensen
Aug 21, 2013

I solved this problem using perfect squares . Let p ( n ) = n 2 p(n)=n^2 be the n n 'th perfect square. If p ( n ) x < p ( n + 1 ) p(n) \leq x < p(n+1) that means that x = n \lfloor\sqrt{x}\rfloor=n . Since x \lfloor\sqrt{x}\rfloor has to be equal to x + 34 \lfloor\sqrt{x+34}\rfloor , we must find two perfect primes, that lie at least 34 34 apart (if they don't, x \lfloor\sqrt{x}\rfloor will be floored towards n n , and x + 34 \lfloor\sqrt{x+34}\rfloor , will be floored towards ( n + 1 ) (n+1) . The first pair of perfect primes with this property is p ( 17 ) = 289 p(17)=289 and p ( 18 ) = 324 p(18)=324 , as their difference is 35 35 . Checking the equation with x = 289 x=289 confirms, that this is a solution.

Moderator note:

Make sure you read your solution to ensure that what you wrote is what you think you wrote.

I'm not sure why it changed from perfect squares into perfect primes . This is likely a typo that could be avoided.

Roberto Vilarrubi
Nov 13, 2014

x must be a perfect square.

Using mean value theorem, for some u, x < u < x+ 34

[\sqrt { x+ 34 }] -\sqrt{x} = 34/(2 \sqrt{u} ) < 1 when \sqrt{u}> 17

Rafe Zayed
Nov 1, 2014

according to the ques , while adding 34 with x , the square root doesnt increase.how is it possible? we know that , the difference between two conjecutive integer is the sum of them. so the least difference should be odd. so , we have to calculate the next odd difference after 34 and that is 35.now how we can get 35? yes , its 18+17.so , root of x+35 should be 18.hence x is 18^2-35 or 289

Akira Sonoda
Aug 23, 2013

Now, bases on the assumptions, we can say that, the difference of the 2 perfect squares is less than 34, hence the square root symbol. So, we need to find 2 perfect squares that have a difference of 34 and/or less. The 2 numbers are 256 and 289. Therefore, the answer is 289.

Now, let's do this algebraically. It's basically y = x 2 ; ( x + 1 ) 2 x 2 34 y = x^2; (x+1)^2 - x^2 \geq 34 . Basically, when solved, we get y = 17, so x = 289.

Saurav Tyagi
Aug 18, 2013

jst see the perfect square after which adding 34 to it stops the result before the next perfect square as it is 289. ... 289+34 is less than 364 that is the next

don't u mean 324 not 364

abhijeet venkataraman - 7 years, 9 months ago

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