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Determine the largest positive integer y y for which there is a positive integer x x satisfying the equation 2 13 + 2 10 + 2 x = y 2 2^{13} + 2^{10} + 2^{x} = y^{2} .


The answer is 160.

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3 solutions

Miles Koumouris
Jan 2, 2017

2 13 + 2 10 + 2 x = y 2 2^{13} + 2^{10} + 2^{x} = y^{2}

2 10 ( 2 3 + 1 ) + 2 x = y 2 \Leftrightarrow 2^{10}(2^{3}+1) + 2^{x} = y^{2}

( 2 5 × 3 ) 2 + 2 x = y 2 \Leftrightarrow (2^{5}\times 3)^2 + 2^{x} = y^{2}

2 x = y 2 9 6 2 \Leftrightarrow 2^{x}= y^{2} - 96^2

2 x = ( y + 96 ) ( y 96 ) \Leftrightarrow 2^{x}= (y + 96)(y-96)

y Z + , y + 96 = 2 m and y 96 = 2 n , { m , n } Z + \because y \in \mathbb{Z}^+, \textrm{ }y+96=2^m \textrm{ and } y-96=2^n, \left \{m,n \right \} \in \mathbb{Z}^+

2 m 2 n = 192 = 2 6 × 3 2^m - 2^n = 192 = 2^6 \times 3

2 m 6 2 n 6 = 3 \Rightarrow 2^{m-6} - 2^{n-6} = 3

m = 8 and n = 6 \Rightarrow m=8 \textrm{ and } n=6

y = 2 8 96 = 2 6 + 96 = 160 . \Rightarrow y=2^8-96=2^6+96=\boxed{160}.

Note that 160 is in fact the only solution for y.

Miles Koumouris - 4 years, 5 months ago
Kushal Bose
Jan 3, 2017

2 10 + 2 13 + 2 x = y 2 = > ( 2 5 ) 2 + 2. 2 5 . 2 7 + ( 2 7 ) 2 + 2 x ( 2 7 ) 2 = y 2 = ( 2 5 + 2 7 ) 2 + ( 2 x 2 14 ) = y 2 2^{10}+ 2^{13}+2^x=y^2 \\ => (2^5)^2+ 2.2^5.2^7 +(2^7)^2 +2^x-(2^7)^2=y^2 \\ =(2^5+2^7)^2+(2^x-2^{14})=y^2

Minimum ocurrs when 2 x 2 14 = 0 = > 2 x = 2 14 2^x-2^{14}=0 => 2^x=2^{14} then y 2 = ( 2 5 + 2 7 ) 2 = > y = 2 5 + 2 7 = 160 y^2=(2^5+2^7)^2 => y=2^5+2^7=160

Why does minimum occur here? What if 2^x<2^14 and y^2 is lower than 160^2? Maybe you could explain that reasoning just a bit more clearly.. Please tell me if I've missed something obvious though!

Miles Koumouris - 4 years, 5 months ago
Zach Abueg
Jan 2, 2017

If we take the square root of both sides to isolate y y and factor 2 10 2^{10} out of the left side, we obtain

( 2 10 ) ( 1 + 2 3 + 2 x ) = y \sqrt {(2^{10})(1 + 2^3 + 2^x)} = y

( 2 10 ) ( 1 + 8 + 2 x ) = y \sqrt {(2^{10})(1 + 8 + 2^x)} = y

( 2 10 ) ( 9 + 2 x ) = y \sqrt {(2^{10})(9 + 2^x)} = y

By using trial and error, we find that the lowest x x for which y y is an integer value is 4 4 , and y = 160 y = 160 .

Edit (as suggested by the asker):

We can see that the power of 2 2 comes out of the square root : :

2 10 = 2 5 \sqrt{2^{10}} = 2^5 .

2 5 9 + 2 x = y 2^5\sqrt{9 + 2^x} = y

For y y to be an integer, there must be an x x that satisfies : :

9 + 2 x = z 2 , z Z 9 + 2^x = z^2, z \in \mathbb{Z}

x = 4 x = 4 is, in fact, the only solution, creating the Pythagorean triple 3 , 4 , 5 : 3, 4, 5:

9 + 16 = 25 9 + 16 = 25

z = 5 Z z = 5 \in \mathbb{Z}

This is great, but since the question asks for the largest value of y, your solution does not necessarily show that 160 must be the answer. I think you could quite easily fix this by adding a proof after your fourth line to show that x=4 is the only solution to 9+2^x=z^2 where z is an integer. Nonetheless, this is a very good technique to quickly reach the answer!

Miles Koumouris - 4 years, 5 months ago

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I definitely agree, it's less rigorous than your solution. Thank you for the suggestion! I'll surely add that in soon, as I'm just about to sleep LOL. Appreciate it Gh!

Zach Abueg - 4 years, 5 months ago

Miles, I was looking back at some of my first solutions (this was, in fact, my third solution ever) and I thought: you know, now that I've been on this site for 6 months and learned so much about problem solving, this would have been a breeze looking in hindsight. I've grown and learned a lot, it's amazing and I'm actually really proud of myself :)

Just a little self-reflection, sorry to have passed that by you haha. Thanks for being one of the very first people to enlighten me and pave that way for me! :)

Zach Abueg - 3 years, 10 months ago

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