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 isosceles triangle with 
. Let 
be a point on 
. Let 
be a point inside the triangle such that 
and![\[
CL \cdot BD = BL \cdot CD.
\]](http://latex.artofproblemsolving.com/1/6/7/16771838c95f86e92f79b8d049e46ab473e6287d.png)
Prove that the circumcenter of triangle 
lies on line 
.
satisfying 
and
for all 
.
be a cyclic quadrilateral with 
being the midpoints of segments 
respectively. Suppose 
is the intersection of diagonals 
of quadrilateral 
Define 
to be the isogonal conjugate point of point in 
Define 
similarly. Suppose 
intersects 
at a point 
Prove that: The Newton-Gauss line of quadrilateral bisects segment 
be an odd prime number. How many -element subsets 
of 
are there, the sum of whose elements is divisible by ?
be a positive integer. Ana and Banana play a game. Banana thinks of a function 
and a prime number 
. He tells Ana that 
is nonconstant, 
, and 
for all integers 
. Ana's goal is to determine the value of . She writes down integers 
. After seeing this list, Banana writes down 
in order. Ana wins if she can determine the value of from this information. Find the smallest value of for which Ana has a winning strategy.
be an acute triangle and 
its orthocenter. Let 
be a point on the parallel through 
to 
such that 
. Define 
and 
as points on the parallels through 
to 
and through 
to 
similarly. If 
are positioned around the sides of as in the given configuration, prove that are collinear.
be an acute triangle with 
, its orthocenter and 
it's circumcenter. Let be a point on the parallel through to such that 
and and are on different sides of . Denote by 
the intersection of the circumcircle of and 
, where 
is the reflection of over , 
the midpoint of 
, the intersection of 
and the parallel through to 
, the intersection of 
and the perpendicular through to 
and 
a point on such that 
, where 
is the midpoint of 
. Prove that 
lie on a line.
is inscribed in a circle 
. Inscribed in a triangle circle touchs the sides in a point . 
— the circle inscribed in a segment 
circle of , and passing through a point . Let points and 
— the centers of circles and an extra inscribed circle (touching side ) respectively. Prove, that lines 
and 
are parallel.
be cyclic quadrilateral. Let and 
intersect at , and let and 
intersect at 
. Let and are points on and such that 
. Let and be the intersections of 
with the diagonals of . Prove that circumcircles of triangles 
and 
are tangent at a fixed point.
be an acute triangle. Points 
, and lie on a line in this order and satisfy 
. Let and be the midpoints of 
and 
, respectively. Suppose triangle 
is acute, and let be its orthocentre. Points and lie on lines 
and 
, respectively, such that 
and are concyclic and pairwise different, and 
and are concyclic and pairwise different. Prove that 
and are concyclic.
in the given equation gives 
Call this statement 
. This immediately gives is injective.
and 
gives 
Here 
is some constant. Also, comparing 
and 
and using injectivity, we have 
Here 
is another constant. Now this gives 
holds for all 
, which forces 
. So in fact 
Now 
gives 
Call this new statement 
.
gives 
Comparing 
and 
and mutilplying y 
gives 
Using 
in each of these terms, we have 
, and replacing 
, we get 
Now we use the statements 
to simplify that into 
Setting 
, there, we get 
, and since 
because of injectivity, we get 
, which in turn implies 
can be written as 
, which becomes Cauchy's equation after setting 
. Since the codomain of is bounded from below, we must have 
, and only 
fits.
It is clear that we can replace by 
and get:
Suppose that 
for some 
Then 
gives us 
contradiction. So 
But this means that
Combining the two boxed results gives us 
such that
for all positive real numbers and 
.
for all 
. Put 
thus 
for all 
. Observe that as 
we see 
has no upper bound. Thus, for any 
it is possible to pick with 
; hence 
is a subset of , consequently is surjective over 
. If 
then plugging 
and 
subsequently, we conclude 
or is injective. Thus, is a bijection on positive reals.
yielding 
for all 
. Swapping 
fixes the LHS, hence 
for some constant and all 
. Hence 
for all . Playing the Devil's trick again, we put 
in the original equation; so 
and swap 
; injectivity of then yields that 
is a constant function. Thus, 
and so 
for all 
. Now plug 
in 
to conclude that 
for all 
. Now we immediately get 
for all and so 
(putting 
); hence is additive too. Now is strictly increasing so if 
then 
and vice-versa. Thus, 
for all 
and is the identity function. It also works. 
to note that is injective and 
. Now take with 
gives 
. Therefore is bijective.
Replace 
by 
in the above relation and consider symmetric equation with 
swapped we deduce
Fix and vary to get using bijective that 
so 
, i.e. 
.
Replace by 
to get
Now use injectivity to get
. Taking a similar consideration as before with 
we see that 
. Therefore 
, so 
becomes Cauchy FE. Extend by 
for all 
and note that extended satisfies Cauchy on the reals, and 
for all 
, so is linear, from which we conclude that 
, as desired.
such thatfor all positive real numbers and .
is the only solution to the given equation. Note that it indeed works.
denote the given assertion.
Therefore, let 
. Then we have 
and some 
.
is injective . Now 
imply that . So 
and hence is bijective .
.
. Then, 
.
. (By induction on this, we get 
and observe that:
Now using 
and 
.
. So , iterating on both sides we get 
. is an involution. Now comparing and 
we see that
Plugging this back in we get 
and 
.
.
, we get unbounded values of , and for a fixed value of , we get that is surjective over 
. So, letting 
tend towards infinity gives surjectivity.
, 
gives 
, so is bijective.
. As varies for fixed , we get that the image of 
is 
. Thus, if 
, 
, so is increasing. As it is both increasing and bijective, our function must be continuous.
. This gives that 
, so it is fixed as varies. Note that we must have 
to make sure the argument is positive, and as approaches this lower bound, the LHS gets arbitrarily close to 
. The RHS cannot be less than , since it will otherwise be exceeded as approaches 
. Likewise, it can't be more than . So, we must have 
. As 
, LHS approaches , so we can get by similar logic that it must always be . Thus, 
.
, which gives 
. As , LHS approaches 
while RHS is always larger, but approaches 
by above. Using a similar argument, if 
, then we eventualy have RHS>LHS, and if 
, we have the opposite. Hence, 
. is both an involution and increasing. If it is nonconstant, though, we can find 
such that 
. Thus, is the only solution.
such that
for all positive real numbers and . is injective function.
the given assertion.
: 
so 
, so is injective.
. is injective we have that 
.
so 
.
so 
.
it's simple to show that 
so 
. we have that 
so 
.
, thus 
. and 
.
etc.
denote the given assertion, by comparing ![\[P\left(\frac{1}{x}, \frac{1}{f(1)}\right) \implies \frac{1+xf\left(\frac{1}{f(1)}\right)}{f\left(\frac{1}{f(1)}\right)}=f(f(1)+f(x))\]](http://latex.artofproblemsolving.com/8/1/1/811b4efc48217026e70946539a8d3ea55150d4b8.png)
and ![\[P\left(1,\frac{1}{f(x)}\right)\implies \frac{1+f\left(\frac{1}{f(x)}\right)}{f\left(\frac{1}{f(x)}\right)}=f(f(x)+f(1))\]](http://latex.artofproblemsolving.com/1/9/8/198edb0bf4d14a9b3080c10c46ecc4ef6a437e0a.png)
we will get 
where 
From here, it's also clear that is injective. Also, by comparing ![\[P(f(x),1) \implies \frac{f(x)+f(1)}{f(x)f(1)}=f\left(1+f\left(\frac{1}{f(x)}\right)\right)\]](http://latex.artofproblemsolving.com/b/c/6/bc6babd50b111a2e007f9b1a01d9e36524efd067.png)
and ![\[P(f(1),x) \implies \frac{f(1)+f(x)}{f(1)f(x)}=f\left(\frac{1}{x}+f\left(\frac{1}{f(1)}\right)\right)\]](http://latex.artofproblemsolving.com/e/7/d/e7dd89e6fa05585f0b6a8978c7df2c4c89855805.png)
we will get 
where 
Therefore, we have 
or equivalently, 
Since this holds for all positive real , it must be the case where 
which implies 
Now notice that ![\[P\left(\frac{1}{x}, \frac{1}{f(x)}\right) \implies f(2f(x))=2x\]](http://latex.artofproblemsolving.com/7/2/f/72f36b565fb2f8b0259131176877377cbd21bd04.png)
and so 
Then, ![\[P\left(\frac{1}{x}, \frac{1}{f(3x)}\right)\implies f(f(3x)+f(x))=4x=f(2f(2x)) \implies f(3x)+f(x)=2f(2x)\]](http://latex.artofproblemsolving.com/2/4/b/24b751b1a41faa66d24c744f27e7432a880a5a01.png)
and ![\[P\left(\frac{1}{2x}, \frac{1}{f(4x)}\right)\implies f(4f(x)+f(2x))=f(f(4x)+f(2x))=6x=f(2f(3x)) \implies 4f(x)+f(2x)=2f(3x)\]](http://latex.artofproblemsolving.com/f/0/6/f0653891e21e5989dcf4f515163269a8c0da583d.png)
give us ![\[4f(2x)-2f(x)=2f(3x)=4f(x)+f(2x) \implies f(2x)=2f(x) \ \ \forall x\in \mathbb{R}^{+}.\]](http://latex.artofproblemsolving.com/1/7/2/1724321d59be8c5e7e5cd64552f1d69de27b4675.png)
Hence, 
and 
Lastly, ![\[P\left(f\left(\frac{1}{x}\right), \frac{1}{y}\right)\implies f(x+y)=\frac{f\left(\frac{1}{x}\right)+f\left(\frac{1}{y}\right)}{f\left(\frac{1}{x}\right)f\left(\frac{1}{y}\right)}=f(x)+f(y)\]](http://latex.artofproblemsolving.com/7/2/f/72fae9afa4e92a0b01b59a72782717e96881952c.png)
implies is additive over the positive reals, thus is linear and letting 
where 
are constants, we see that since 
, 
is the only solution. 
Now put instead of in equation. Let 
.
then 
implies that is injective.
Let 
be .
which holds for any 
which implies that is linear so 
.
which since are constant but 
are not implies that 
so 
so 
which clearly works.
, which also implies injective as a result
, we get 
for some constant over all values of 
, so 
implies 
, we have 
gives us 
, Since is surjective, varying the value of , we have that is a strictly increasing function
where 
Then, note 
contradiction. The same holds for when 
for all values of 
such thatfor all positive real numbers and . be the assertion of the given FE, which is 
Assume that , then 
and 
give or is injective.
and 
, we easily get 
Similarly, comparing 
and 
, we get 
for all . Let , we get 
, so 
and gives 
gives 
Plugging by 
into the above FE and changing the role of , by the injectivity, we get 
If 
, 
and the injectivity give 
, absurb. Thus 
, so 
gives 
immediately implies is additive, so after checking, we get for all is a solution.
. is bijective.
. For surjectivity, observe we can hit any value above 
, since we can make as small as we want we are done.
, we have 
as is a positive real. Then we have 
, so letting approach zero gives 
. Now substitute 
, so we have 
for 
. Now observe each pair 
results in exactly one value of , which is the same as the value of given by 
, but also this value is uniquely determined by so is constant, thus 
, but since the left hand side gets as small as we want we must have and 
.
, then we have 
, so 
giving 
. is the identity. Assume 
but 
. Now we have 
, giving 
. Clearly, 
cannot be 
, so 
for 
. Likewise, assume 
, which gives 
, so iff 
. This fact carries the rest of the solution, we can now proceed with a standard rational extension. over rationals. Let be the assertion 
. Now 
gives 
, then we can use the assertion , being 
to get 
. Now we can always induct, do 
to get 
and 
to get 
. Now to get all rationals we induct on the denominator, we can use 
to get all rationals with denominator , then use 
and 
to get all rationals with denominator 
, and so one and so forth we win.
for all reals. First take 
. Assume 
. Then there exists some rational 
with 
, contradiction. Symmetrical argument proves for . Now consider 
, this gives 
, giving 
by injectivity.
![\[\frac{1}{f(y)}+x=f(\frac{1}{y}+f(x))\]](http://latex.artofproblemsolving.com/d/6/1/d61b799f90102cf14a4dda741b04b6fd11dfb855.png)
Answer is which holds. Let be the assertion. is bijective., compare 
and 
to get contradiction. Fix and increase to see that takes all sufficiently large positive values. We can pick 
sufficiently small thus, is surjective..
in order to observe that![\[\frac{1}{f(\frac{1}{f(y)})}+x=f(f(x)+f(y))=\frac{1}{f(\frac{1}{f(x)})}+y\implies x-\frac{1}{f(\frac{1}{f(x)})}=y-\frac{1}{f(\frac{1}{f(y)})}\]](http://latex.artofproblemsolving.com/e/0/e/e0ec6817e4de007d6120947e82374c8453bdbf2a.png)
Since this implies 
is constant and it's smaller than any positive , it must be a nonpositive constant. Let 
or 
where 
. By symmetry and injectivity we have![\[f(\frac{1}{x}+\frac{1}{y+c})=f(\frac{1}{x}+f(\frac{1}{f(y)}))=\frac{1}{f(y)}+\frac{1}{f(x)}=f(\frac{1}{y}+f(\frac{1}{f(x)}))=f(\frac{1}{y}+\frac{1}{x+c})\]](http://latex.artofproblemsolving.com/0/2/2/022f9c23f19b6522c4a79de2defeed855d4c8212.png)
Thus, 
is constant which requires . So 
as we have claimed. is involution and 
. gives 
. We get 
and injectivity implies 
is independent of . Let 
. Since 
we observe 
. Thus, 
.![\[f(f(x))+f(\frac{1}{y})+d=f(f(x)+\frac{1}{y})=x+\frac{1}{f(y)}\]](http://latex.artofproblemsolving.com/6/6/2/662168d5b320e079781cb5a5f7d3c51f3fe62441.png)
and 
. Since 
, we must have 
which implies 
. Thus, is an involution and . is additive.
yields 
. is additive and takes values on positive reals, is Cauchy function hence which implies 
. So is the only solution as desired.
Prove that
Prove that
Prove that
,
.
,
forall 
forall 
,
,
,
,
(from here, we easily get that: 
)
,
,
,
,
,
,
,
,
and let 
.
.![$P(x,\frac{1}{f(y)})\implies f(f(x)+f(y))=x+y\implies [a+b=c+d\iff f(a)+f(b)=f(c)+f(d)]\implies \exists g:\mathbb{R}\rightarrow \mathbb{R},g(a)=f(x+a)-f(x)$](http://latex.artofproblemsolving.com/7/5/8/7580abc760b6c61ee8fad23138f5e465924202db.png)
.
.
and so 
,
.
which clearly satisfied the condition.