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Solve for the rational numbers x and y: 2x+y3xy62x+2y=72.

 Jul 13, 2019
 #1
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Using Newton-Raphson method, it gives the following solution:

x = 1/2   and     y=1/2

 Jul 13, 2019
 #2
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2x+y3xy62x+2y = 72 2x+y3xy62(x+y) = 72 2x+y3xy(62)x+y = 72 2x+y3xy36x+y = 72 2x+y36x+y3xy = 72 (236)x+y3xy = 72 72x+y3xy = 72

 

At this point we can see that the equation will definitely be satisfied if

 

x + y  =  1    and

x – y  =  0

 

So    x  =  1/2    and    y = 1/2    is definitely a solution.

 

But that doesn't necessarily mean it is the only solution.

 

I don't know how to prove that it is the only rational solution, but it does seem to be.

 

Maybe someone else can explain why that is.

 Jul 13, 2019
 #3
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LHS:

2x+y.3xy.62x+2y=2x+y.3xy.(2×3)2x+2y=2x+y.3xy.22x+2y.32x+2y=23x+3y.33x+y

 

RHS:

72=23.32

 

It's an identity, so equating indices,

3x+3y=3,3x+y=2.

 

Solve, x = 1/2, y = 1/2.

 

The solution is unique.

 Jul 14, 2019
 #4
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The expression p1Q1*p2Q2*....*pnQn for p1,....,pn distinct primes and Q1,....,Qn rational numbers will result in an integer if and only if Q1,...,Qn are all nonnegative integers

 Jul 14, 2019

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