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.
and 
.
?
be triangle and point 
be 
altitude of 
.
be a point on 
and 
such that 
and 
. Point 
is the intersection of 
and 
. Point 
be intersection of 
and 
. Line 
intersect circumcircle of again at 
.
.
bisect .
, define a sequence of integers 
by letting 
and 
for 
. Let 
. Determine the largest possible 
such that, for some positive integer , the numbers 
are all prime.
cities in a country, where 
is an integer. Some pairs of cities are connected by direct (two-way) flights. For two cities 
and 
we define:
A 
between and as a sequence of distinct cities 
, 
, such that there are direct flights between 
and 
for every 
;
A 
between and as a path between and such that no other path between and has more cities;
A 
between and as a path between and such that no other path between and has fewer cities. and in the country, there exist a long path and a short path between them that have no cities in common (except and ). Let 
be the total number of pairs of cities in the country that are connected by direct flights. In terms of , find all possible values 
such that 
for all real number 
and 
be a sequence of perfect squares such that 
can be obtained by concatenating a digit to the right of 
. Determine all such sequences that are of maximum length.
be unit vectors on a plane, where is an odd number. Prove that there exist 
for 
such that 
and the point 
on the side . The line 
intersects the line at point , and the line 
intersects the line 
at point . Prove that the lines 
and 
are perpendicular.
. Then we get 
.
, we know BG and DF are perpendicular.
; according to Blanchet theorem, is bisector of 
, but by symmetry 
, while 
, thus 
, hence 
.
.
a
.
and 
Prove that
and 
Prove that
. Prove that
