integral solutions

Prove that for n ≥ 6, the equation

1x12 + 1x22 + 1x32 + .... + 1x2n = 1 has integral solutions.

10 Answers

1
Euclid ·

???

341
Hari Shankar ·

If someone has the patience to find solutions for x=6,7,8, then there is a simple solution possible.

Say we have a solution for n=k,

\frac{1}{x_1^2} + \frac{1}{x_2^2}+...+\frac{1}{x_k^2} =1

Then \frac{1}{4x_1^2} + \frac{1}{4x_2^2}+...+\frac{1}{4x_k^2} =\frac{1}{4}

and so

\frac{1}{4x_1^2} + \frac{1}{4x_2^2}+...+\frac{1}{4x_k^2} + \frac{1}{4} + \frac{1}{4} + \frac{1}{4} =1

i.e. we have the solution (2x_1, 2x_2,...,2x_k,2,2,2) for n = k+3.

23
qwerty ·

[5] prophet sir u r genius

341
Hari Shankar ·

Turns out, the solutions for the three cases are easy to set up (in other words i was plain lazy)

n=6:

\left(\frac{1}{4} + \frac{1}{9} \right)+ \left(\frac{1}{4} + \frac{1}{9} \right)+\left(\frac{1}{4} + \frac{1}{36} \right)=1

n=7:

\frac{1}{16}+\frac{1}{16}+\frac{1}{16}+\frac{1}{16}+\frac{1}{4}+\frac{1}{4}+\frac{1}{4} = 1

n=8:

\left(\frac{1}{4} + \frac{1}{9} \right)+ \left(\frac{1}{4} + \frac{1}{9} \right)+\left(\frac{1}{9} + \frac{1}{36} \right)+\left(\frac{1}{9} + \frac{1}{36} \right) = 1

1
Euclid ·

thank u sir!

11
Devil ·

We have an alternative soln if the multiplication by 1/4 does not hit.

Recollect the fact that a square can be dividen in n non-overlapping squares for all n≥6.

Take the unit squares.

If we say 1xi=aii.

We see each ai is either of the form 12k, where k are non negative integers. Basically these ais arethe lengths of the squares into which the unit square has been divided.

The fact that the above qn has integral solutions follow from the fact that since these squares are non-overlapping, their sum of areas must be equal to the area of the square which was partitioned, i.e 1.

341
Hari Shankar ·

Heck ya! The connection with that prob though nagging at me did not occur this clearly to me.

The problem referred to is this: http://targetiit.com/iit-jee-forum/posts/dividing-squares-12381.html

23
qwerty ·

Cant open the link [2]

341
Hari Shankar ·

copying and pasting the link is working for me

23
qwerty ·

yeah its opening now ( i had blocked tiit on my pc due to addiction [4] )

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