Difference between revisions of "1976 USAMO Problems/Problem 3"
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− | Rewrite the equation into <math>a^2+b^2-a^2b^2=-c^2</math> or <math>a^2+b^2-2a^2b^2+a^2b^2 = -c^2</math>. The LHS of this equation can be written as the sum of two squares, i.e., <math>(a-b)^2+(ab)^2 = -c^2</math>. It is easy to see that LHS <math>\leq 0</math> and RHS <math>\geq 0</math>. Thus, both sides must be equal to zero: <math>(a-b)^2+(ab)^2 = 0</math> and <math>c^2 = 0</math>. From there we conclude that <math>a=b=c=0</math>. | + | Rewrite the equation into <math>a^2+b^2-a^2b^2=-c^2</math> or <math>a^2+b^2-2a^2b^2+a^2b^2 = -c^2</math>. The LHS of this equation can be written as the sum of two squares, i.e., <math>(a-b)^2+(ab)^2 = -c^2</math>. It is easy to see that LHS <math>\leq 0</math> and RHS <math>\geq 0</math>. Thus, both sides must be equal to zero: <math>(a-b)^2+(ab)^2 = 0</math> and <math>c^2 = 0</math>. From there we conclude that <math>a=b=c=0</math> is the only solution. |
== See Also == | == See Also == | ||
{{USAMO box|year=1976|num-b=2|num-a=4}} | {{USAMO box|year=1976|num-b=2|num-a=4}} |
Revision as of 23:57, 9 October 2020
Problem
Determine all integral solutions of .
Solution
Either or
. If
, then
. Symmetry applies for
as well. If
, then
. Now we look at
:
: Since a square is either 1 or 0 mod 4, then all the other squares are 0 mod 4. Let
,
, and
. Thus
. Since the LHS is divisible by four, all the variables are divisible by 4, and we must do this over and over again, and from infinite descent, there are no non-zero solutions when
.
: Since
,
, and
. But for this to be true,
, which is an impossibility. Thus there are no non-zero solutions when
.
Thus the only solution is the solution above: .
Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.
Rewrite the equation into or
. The LHS of this equation can be written as the sum of two squares, i.e.,
. It is easy to see that LHS
and RHS
. Thus, both sides must be equal to zero:
and
. From there we conclude that
is the only solution.
See Also
1976 USAMO (Problems • Resources) | ||
Preceded by Problem 2 |
Followed by Problem 4 | |
1 • 2 • 3 • 4 • 5 | ||
All USAMO Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.