Cubes over, now are squares!

Given 2 x + 2 13 + 2 10 2^{x} + 2^{13} + 2^{10} is a perfect square number. Determine the value of x x .

Try out Squares over, now are cubes!


The answer is 14.

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3 solutions

Abhisek Mohanty
Apr 3, 2015

You know that a 2 + b 2 + 2 a b = ( a + b ) 2 a^{2} + b^{2} + 2ab = (a+b)^{2}

Now this is in the form of a 2 + b 2 + 2 a b a^{2} + b^{2} + 2ab

Noe it is clear that 2 13 2^{13} can't be a 2 a^{2} or b 2 b^{2} , because in a 2 a^{2} or b 2 b^{2} the power is even but in the other it is odd. So it is clear the 2 13 a 2 b 2 2^{13} \neq a^{2} \neq b^{2}

Now let us take a 2 = 2 10 a^{2} = 2^{10} a = 2 5 a= 2^{5}

Therefore , 2 13 = 2 a b 2^{13} = 2ab

or, 2 13 = 2 × 2 5 × b 2^{13} = 2\times 2^{5}\times b

or, 2 13 = 2 6 × b 2^{13} = 2^{6}\times b or 2 7 = b 2^ {7} = b

Now, b = 2 7 b= 2^{7}

But here as I have mentioned in the first that the given expression is expressed in the expanded form of ( a + b ) 2 (a+b)^{2}

Therefore, 2 x = b 2 2^{x} = b^{2}

or, 2 x = 2 7 × 2 2^{x} = 2^{7\times 2}

or, 2 x = 2 14 2^{x}= 2^{14}

or, x = 14 x=14

if you like this please F O L L O W \boxed{FOLLOW} me !!!!!!!!

It's not clear to me why 2 13 2^{13} must be equal to a 2 a^2 , b 2 b^2 , or 2 a b 2ab . Most integers can be written as a sum of three terms in lots of different ways. I think it would help if you could be more specific when you introduce a and b . What are they? Are they just two integers that add up to the number being squared? Could they be any two such integers?

Matt Enlow - 6 years, 2 months ago
Sunil Pradhan
May 5, 2016

2^x + 2^13 + 2^10

= 2^10(2^x-10 + 2^3 + 1)

= 2^10(2^x-10 + 9)

(2^x-10 + 9) must be a square number out of this 9 is square number

to get next square number 27 add 16 to 9

so 2^(x-10) = 16 = 2^4

then x – 10 = 4 so x = 14


Actually. I used a little bit of "trial and error" thingy...... Let us assume x=10 (Case i) X<10 (Case ii) or, x>10 (Case iii). Clearly, x ≠ 10 (Substitute x = 10 and take 2^10 common, we don’t get factors for a perfect square… Case ii: x<10 Given question simplified: 2^10 [1 + 8 + 2^(10-x)] If x<10 and even, Given question would be, 2^x (2^(13-x) + 2^(10-x ) But 13 –x and 10 –x can only result in a combination of odd and even numbers.. So, we can’t have a perfect combination of even numbers. SO, x can’t be even Similarly, if x is odd, 13 –x and 10 –x can only result in a combination of even and odd numbers. So, x can’t be odd either. Therefore, x can be only greater than 10. (Case (iii) is only right) Given simplified question: 2^10 [(9 + 2^(x-10 )] 2^10 is already a perfect square. So, 9 + 2^(x-10) is a perfect square. So, That gives x = 14. (9 +16 = 25, 25 is a perfect square) X =14 is the ONLY POSSIBLE ANSWER

Sorry for the congestion of the reply.. I am in a bit of a hurry

Ñ Prâvéëñ Çhãñdhär - 6 years, 2 months ago

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And, I can't even bring up a nice organisation of my answer, like Abhishek did !!!! :/

Ñ Prâvéëñ Çhãñdhär - 6 years, 2 months ago

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No,Your answer is quiet good

Harshi Singh - 6 years, 2 months ago

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