AoPS Academy
[/quote]
, 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.
be an integer, and let 
be the set ![\[A_n = \{2^n - 2^k\mid k \in \mathbb{Z},\, 0 \le k < n\}.\]](http://latex.artofproblemsolving.com/f/9/9/f99d070ae4a6efc4812db4997ec9385542512c2a.png)
Determine the largest positive integer that cannot be written as the sum of one or more (not necessarily distinct) elements of .
. Prove that
Let 
. Prove that
Let 
. Prove that
Let 
. Prove that
![$P(x) \in \mathbb{R}[x]$](http://latex.artofproblemsolving.com/4/5/3/453a624c3b002c1b0e78e0023b24dd22ddd03557.png)
such that:
with 
is an 
th prime and 
is an arbitrary positive integer.
denote the number of ordered triples of positive integers 
satisfying![\[
x + yz + zx = n.
\]](http://latex.artofproblemsolving.com/c/d/9/cd97547db7950abad749b1aed29bb288e15dcd53.png)
and 
.
denote the number of positive divisors of 
. Express in terms of .
of acute triangle 
is 
. The ray drawn from through the circumcenter 
intersects 
at 
. Let the midpoint of 
be 
. Define 
, 
, 
, 
, 
, 
similarly. Prove that 
, 
, 
are concurrent.
such that 
is a perfect square?
other knights where friendship is mutual. Prove that King Arthur can place some of his knights around a round table in such a way that every knight is friends with the 
people adjacent to him and between them there are at least 
friendships of knights that are not adjacent to each other.
and 
intersect at 
. Diameter 
of 
intersects 
at 
, Diameter 
of intersects at 
. 
intersects at 
, 
intersects at 
. 
. Prove that 
.
and intersect at . Bisector of outer angle 
intersects at 
, at 
. 
is a point on 
, 
. Prove that 
.
be positive reals such that 
. Prove that the inequality![\[a \sqrt[3]{1+b-c} + b\sqrt[3]{1+c-a} + c\sqrt[3]{1+a-b} \leq 1\]](http://latex.artofproblemsolving.com/f/2/d/f2db8f577625a01bcfaceaaae3fd6651b370ae80.png)
that verfy the following equation :
is the 
-excenter of the triangle and 
is the circumcircle of this triangle. 
is the middle of arc of which doesn't contain . 
meets at 
. Prove that
and 
be the incenter and the excenters of 
againts 
Since is 9-point circle of 
then cuts 
at their midpoints 
Since 
(
bisects 
), then 
are concylic hence
which is due to 
bisecting 
than 
be the midpoint of arc containing 
is the circumcenter of 
and 
are tangent to 
is the symmedian of 
lies on the symmedian such that 
bisects 
and the conclusion follows
and since 
is an angle bisector of , 
. Now, let be the second intersection of the circumcircle 
of 
and , so we have 
, which means that 
and 
is tangent to , so 
, so by AA symmetry, 
, and the result follows. be the incenter of . Show that it is possible to construct a right triangle with side lengths 
.
is a triangle with circumcircle 
. Let be the incenter, be the midpoint of arc not including , and let 
be the midpoint of arc 
. If 
, show that 
: Let 
. It's trivial by angle chasing that 
and that 
, now PoP kills it.
be the midpoint of arc 
of and let 
be the -excenter. It is well-known that 
are inscribed in the circle 
of diameter 
with center 
Moreover, note that is the midpoint of arc 
, implying that and 
are the internal and external bisectors of , respectively. Therefore, the inversion with power 
combined with a reflection in swaps and fixes lines and 
It follows that the center of 
(the image of ) is a point on line However, the center of must also lie on the perpendicular bisector of 
, implying that the center is itself. Therefore, is fixed under this inversion, and hence is fixed as well. Thus, 
as desired. 
be the midpoint of arc . And 
is a circle with centre and radius 
. 
.
and 
so 
and it's enough to show that 
We know that 
and 
are tangent to . So we can simply show that 
and .
and 
.It's easy to find the coordinates of by intersecting and .So we finish problem using Distance formula.
and .It's easy to find the coordinates of by intersecting and .So we finish problem using Distance formula.
and .It's easy to find the coordinates of by intersecting and .So we finish problem using Distance formula.
are both tangent to circumcircle of 
by angle chasing.Also 
is cyclic so is the midpoint of symmedian in triangle 
and so two triangles 
is similar to 
from which the relation follows.
is unit circle, 
. So 
and 
. We now want 
. But its easy just solve 
.We get 
And now we need to prove 
. After substituting and 
we see that both sides are equal to 
and we are done.
is the 
dumpty point of 
which implies the result since the dumpty point is the center of spiral similarity sending 
to 
.symmedian and 
, where is the circumcenter of . Now by Ceva's theorem
Since 
,
which implies that 
is a symmedian. Moreover,
Hence 
is cyclic which completes the proof.
since 
bisects 
.![[asy] size(5cm); defaultpen(fontsize(10pt));
pair B,A,C,I,M,NN,IB,T,X; B=dir(110); A=dir(210); C=dir(330); I=incenter(A,B,C); M=extension( (B+C)/2,origin,A,I); NN=extension( (C+A)/2,origin,B,I); IB=2NN-I; T=2*foot(origin,M,IB)-M; X=2*foot(origin,IB,C)-C;
draw(circumcircle(A,C,IB),gray); draw(M--A,gray); draw(B--IB,dashed); draw(C--IB,dashed); draw(unitcircle); draw(A--B--C--cycle); draw(M--IB);
dot("\(A\)",A,A); dot("\(B\)",B,B); dot("\(C\)",C,C); dot("\(M\)",M,M); dot("\(N\)",NN,SW); dot("\(I_B\)",IB,S); dot("\(T\)",T,SE); dot("\(I\)",I,NW); [/asy]](http://latex.artofproblemsolving.com/8/4/7/847e9f4a99fe59c94afa43395984b741d9adc254.png)
![\[\measuredangle I_BTC=\measuredangle MTC=\measuredangle IAC=\measuredangle BI_BC=\measuredangle BI_BT+\measuredangle TI_BC,\]](http://latex.artofproblemsolving.com/4/8/3/483a92707c808d23d140d8b38a45f7729f7c528a.png)
thus 
, so ![\[\triangle TBI_B\sim\triangle TI_BC.\]](http://latex.artofproblemsolving.com/a/5/2/a52d24927e419a6ef011b4a1e19a95c5c22c1f69.png)
It follows that 
, as needed
is the midpoint of the arc, then but notice we want 
and thus we just need one more angle to prove 
. We'll show 
, but 
and as 
and thus 
, proving our claim.
and are tangent to 
gives that is 
dumpty point so this implies the conclusion as
satisfying
Prove
and 
Show that
Where 
Where 
and 
,
.
.
.
.
.
gives 
,
so 
let 
intersect 
so 
combining with 
we are done because we have 
.
is diameter of 
so 
and it's well known that 
are tangent to 
centered at 
and 
)
point in triangle 
so Done 
And that would obviously imply the desired.
Also
And
Whence
So
.
We obtain that 
and
hence we conclude that 
.
follows directly from 
are similar, which clearly implies the result. Relabel 
and let 
be the arc midpoint of minor arc 
so 
.
.
,
.
by standard angle facts. So 
,
.
not containing 
is a kite, we get that 
and we know that 
.
.
.
,
.
.
by 
.
.
are tangents to 
so 
is harmonic.Say 
is the midpoint of 
.
,
So 
and is the spiral center taking 
or,
