SOLUTION: How many natural numbers x and y are there such that {{{1/sqrt(x)+1/sqrt(y) = 1/sqrt(20)}}}

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Question 1062344: How many natural numbers x and y are there such that
Found 2 solutions by ikleyn, Edwin McCravy:
Answer by ikleyn(52776)   (Show Source): You can put this solution on YOUR website!
.
How many natural numbers x and y are there such that
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Answer. How many ? - Zero.
              In other words,  the given equation has no solutions in natural numbers.

Proof

Let "x" and "y" be the solution. Then   (after multiplication both sides by  ) you will get


 =    --->       ( Divide both sides by  )

 = ,   or         ( after introducing the new variable t =  )

 = t   --->    = t   --->   =   --->  t =   --->  t =   --->  t = .


Then from one side,  =  is a rational number.
From the other side,   is an irrational number.

Contradiction.

The contradiction proves the original statement.

The proof is completed.


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Comment from student: Thanks for sparing time for me. But one step of your answer is wrong.
But you accidentally wrote it as That was the part you missed. Anyway thank you.
-------------------


My response. Thank you for your feedback. You are right, I was wrong. My mistake.

I just saw it from the solution by Edwin.

Thank you again, and thanks to Edwin for creating right solution, which is perfect.



Answer by Edwin McCravy(20054)   (Show Source): You can put this solution on YOUR website!


There are 2 solutions in integers.  [3 if you count 
interchanging the x and y values on one of them as 
a separate solution.]





Let , and 





Multiply through by 

















The smallest integer for "a" that will give
a positive value for b is a=3

If , 

If , 

If ,   <--not an integer.

If ,  

That's all the solutions which are integers,
because  is a decreasing function
from b=3 approaching b=2 asymptotically.

Since , and 

Substituting a=3, b=6 , and 

Substituting a=4, b=4 , and 

Substituting a=6, b=3 , and 

There are 2 solutions, (x,y) = (45,180), (x,y) = (80,80)
There are 3 if you count (x,y) = (180,45) as a separate solution
from the first one.

-------
Checking:

1. x = 45, y = 180











That shows that it is a solution.
  
x = 80, y = 80









That shows that it is also a solution.

Edwin

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