Determine the largest positive integer y for which there is a positive integer x satisfying the equation 2 1 3 + 2 1 0 + 2 x = y 2 .
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Note that 160 is in fact the only solution for y.
2 1 0 + 2 1 3 + 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 1 4 ) = y 2
Minimum ocurrs when 2 x − 2 1 4 = 0 = > 2 x = 2 1 4 then y 2 = ( 2 5 + 2 7 ) 2 = > y = 2 5 + 2 7 = 1 6 0
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!
If we take the square root of both sides to isolate y and factor 2 1 0 out of the left side, we obtain
( 2 1 0 ) ( 1 + 2 3 + 2 x ) = y
( 2 1 0 ) ( 1 + 8 + 2 x ) = y
( 2 1 0 ) ( 9 + 2 x ) = y
By using trial and error, we find that the lowest x for which y is an integer value is 4 , and y = 1 6 0 .
Edit (as suggested by the asker):
We can see that the power of 2 comes out of the square root :
2 1 0 = 2 5 .
2 5 9 + 2 x = y
For y to be an integer, there must be an x that satisfies :
9 + 2 x = z 2 , z ∈ Z
x = 4 is, in fact, the only solution, creating the Pythagorean triple 3 , 4 , 5 :
9 + 1 6 = 2 5
z = 5 ∈ 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!
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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!
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! :)
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2 1 3 + 2 1 0 + 2 x = y 2
⇔ 2 1 0 ( 2 3 + 1 ) + 2 x = y 2
⇔ ( 2 5 × 3 ) 2 + 2 x = y 2
⇔ 2 x = y 2 − 9 6 2
⇔ 2 x = ( y + 9 6 ) ( y − 9 6 )
∵ y ∈ Z + , y + 9 6 = 2 m and y − 9 6 = 2 n , { m , n } ∈ Z +
2 m − 2 n = 1 9 2 = 2 6 × 3
⇒ 2 m − 6 − 2 n − 6 = 3
⇒ m = 8 and n = 6
⇒ y = 2 8 − 9 6 = 2 6 + 9 6 = 1 6 0 .