AoPS Academy
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, do not answer with "
is a solution" only. Either you post any kind of proof or at least something unexpected (like "
is the smallest solution). Someone that does not see that is a solution of the above without your post is completely wrong here, this is an IMO-level forum.
such that for all 
,![\[xf(x+f(y))=(y-x)f(f(x)).\]](http://latex.artofproblemsolving.com/e/4/a/e4a3bbb8b91d2aa62d699c24df342fa59be71915.png)
, let 
be the number of all positive divisors of . Find all positive integers such that 
.
of real numbers that satisfy the system of equations![\[ \begin{cases}x^3 = 3x-12y+50, \\ y^3 = 12y+3z-2, \\ z^3 = 27z + 27x. \end{cases}\]](http://latex.artofproblemsolving.com/d/4/d/d4d6c485b4653577e732ff235448bb779914a576.png)
be an acute triangle with orthocenter 
, and let 
be a point on the side 
, lying strictly between 
and 
. The points 
and 
are the feet of the altitudes from and , respectively. Denote by 
is the circumcircle of 
, and let 
be the point on such that 
is a diameter of . Analogously, denote by 
the circumcircle of triangle 
, and let 
be the point such that 
is a diameter of . Prove that 
and are collinear.
such that for every 
, 
, 
equals the number of times appears in the sequence.
be the sequence 
so that 
for each 
and so that the decimal number 
is divisible by 
for each 
. Show that the decimal 
is irrational.
points on the plane, no three of which are collinear, and some set of line segments between them. We say that a subset of these line segments is called a "pairing" if every one of these () points is the endpoint of exactly one of the chosen line segments (in other words, a pairing is a perfect matching).
, there exists such an arrangement of points and line segments (for some value of ) such that there are exactly 
distinct (but not necessarily disjoint) pairings.
be a polynomial with real coefficients such that 
for all 
. Prove that 
for some real constant 
and 
.
be polygons, no two of which are similar. Show that there are polygons 
where 
is similar to 
so that for no 
does contain a polygon that's congruent to 
. so that is wholly contained in , and so it does not intersect the edges of at all.
![\[ 2^n -2, \;\; 3^n -3, \;\; 4^n -4, \;\; 5^n-5, \, \ldots \ldots \]](http://latex.artofproblemsolving.com/d/b/1/db17ef301bf3fd01f8b310f91a14b9dee045e11f.png)
is 
. Also, are there infinitely many n for which the greatest common divisor is 
?
, define:
as its circumcentre, as its orthocentre and 
as its nine-point centre.
, 
and 
to be the midpoint of the smaller arcs.
as the isogonal conjugate of the Nagel point (i.e. the exsimillicenter of the incircle and circumcircle)
as the ninja point of 
as the ninja point of the contact triangle
Points , and 
are collinear, that is is the Feuerbach point.
Points , and are collinear
and 
.Find the minimum of ![$S=\sqrt[3]{\frac{a^3+b^3+c^3}{3}}+\frac{a+b+c}{9}$](http://latex.artofproblemsolving.com/9/d/9/9d93ff9d9f7355e452f1007aeb87423e3491353a.png)
where 
.
be a function such that 
.
.
,
is irraational . The symbols have usual meanings.
and I'm stuck with this. 
Anyone?
irrational.Now if we prove that 
is irrational is over.