311. Some problems from USAMO and other contests.
by Virgil Nicula, Aug 17, 2011, 1:47 AM
USAMO 1979. Find for a fixed
the points
and
so that
and
is maximum.
Proof 1 (trigonometric). Denote
, where
. Apply the Sinus' theorem in the triangles:
. Since
and
,
are constant obtain that
is maximum
is maximum

is maximum
is maximum because

is maximum
is maximum
.
Proof 2 (trigonometric). Denote
. Apply the Sinus' theorem in the triangles:

.
In conclusion,
is maximum
is maximum
.
Proof 3 (synthetic). Denote
. Since
.
In conclusion,
is maximum
is minimum (
is a fixed point and
is a mobile point )
.
Remark. The area
is minimum
the fixed point
is the midpoint of the segment
. Indeed, denote 
and
. Thus, the area
is minimum
is minimum. Observe that
(constant). In conclusion
is minimum
, i.e.
is the midpoint of
.
Can easily construct the minimum position of the line
. Indeed, construct
so that
, where
and
.
USA TST 2010/11. Let
be a triangle. Its incircle touches the sidelines
,
,
at the points
,
,
respectively and its exincircles
touch the sidelines
,
,
at the points
,
,
respectively. Prove that
and
, where
.
I"ll use in my proof some remarkable identities which are here with their proofs.
.
.

. Otherwise,
remark that
.
Proof of proposed problem. Denote the projection
of
on the line
. Therefore,

.


because
.
USAJMO - Maryland. Prove that
.
Proof. On the one hand
and on the other hands

, where 
, what is truly. In conclusion, the inequality
is truly and so from the relations
and
obtain the required inequality.
Remark.
. But
. Indeed, ![$(a+b)^4=\left[\left(a^2+b^2\right)+2ab\right]^2=$](//latex.artofproblemsolving.com/5/7/4/5748ae64d6f9a6be8003653d3cc82d56a25e798e.png)
.
IMO 2003. Let an isosceles
with
, incentre
and the circumcircle
of the triangle
. Let
lying inside
. The lines through
parallel to
and
meet
at
and
respectively. The line through
parallel to
meets
at
respectively. Prove that
and
intersect on the circumcircle of 
Commentary. The enunciation of this problem conceals vainly in its debut that the circumcircle of
is in fact the circle with the diameter
where
is the
- exincenter of 
Proof. Denote : the circumcircle
of
; the circle
with the diameter
; the circumcircles
,
of the isosceles trapezoids
,
respectively ;
the second intersection
between
and the circle
; the intersection
Prove easily that
,
are the tangents from
to the circle 
From the relation
obtain
i.e. the quadrilaterals
,
are similarly as
Thus,


Variation on the same theme. Let a fixed isosceles
where
and let mobile
so that
and exists 
for which
Construct
for which
Find the locus of the intersection
Answer.





Proof 1 (trigonometric). Denote












is maximum





is maximum





Proof 2 (trigonometric). Denote






In conclusion,




Proof 3 (synthetic). Denote



In conclusion,






Remark. The area
![$[XOY]$](http://latex.artofproblemsolving.com/3/6/0/360e7de81b1f6a34e7e7e5e386f830ccc3ac32bf.png)


![$[XY]$](http://latex.artofproblemsolving.com/b/d/5/bd5db5e85aa6daea3eebecaea5d26721edd15203.png)

and
![$[OCP]=[ODP]=\sigma\ ,\ [XDP]=u\ ,\ [YCP]=v$](http://latex.artofproblemsolving.com/6/b/3/6b325f267e4d3929094b09b9143b8ac496e1581e.png)
![$[XOY]$](http://latex.artofproblemsolving.com/3/6/0/360e7de81b1f6a34e7e7e5e386f830ccc3ac32bf.png)








![$[XY]$](http://latex.artofproblemsolving.com/b/d/5/bd5db5e85aa6daea3eebecaea5d26721edd15203.png)
Can easily construct the minimum position of the line





USA TST 2010/11. Let







touch the sidelines









I"ll use in my proof some remarkable identities which are here with their proofs.

















remark that




Proof of proposed problem. Denote the projection
![$\mathrm{pr}_{d}([XY])$](http://latex.artofproblemsolving.com/e/7/f/e7f70aae4a23e1a8481ba4f88429dfea6f658e18.png)
![$[XY]$](http://latex.artofproblemsolving.com/b/d/5/bd5db5e85aa6daea3eebecaea5d26721edd15203.png)

![$\blacksquare\ EF\ge \mathrm{pr}_{BC}([EF])=\mathrm{pr}_{BC}([EA]+[AF])=$](http://latex.artofproblemsolving.com/2/c/e/2ce0660628d9f000bed6d1b1b87a79eea3101523.png)






![$\blacksquare\ E'F'\ge \mathrm{pr}_{BC}([E'F'])=a-\mathrm{pr}_{BC}(BF'+CE')=$](http://latex.artofproblemsolving.com/b/9/5/b959d980df164ce6f79f2fffe32bfec71b9a845f.png)

![$\sum E'F'\ge \sum\left[a-(s-a)(\cos B+\cos C)\right]=$](http://latex.artofproblemsolving.com/e/c/e/ece8ae0ff0b139fe01f0f638de906791c0689bf2.png)






USAJMO - Maryland. Prove that

Proof. On the one hand












Remark.


![$(a+b)^4=\left[\left(a^2+b^2\right)+2ab\right]^2=$](http://latex.artofproblemsolving.com/5/7/4/5748ae64d6f9a6be8003653d3cc82d56a25e798e.png)
![$\left[\left(a^2+b^2\right)^2+4a^2b^2\right]+4ab\left(a^2+b^2\right)\ge$](http://latex.artofproblemsolving.com/1/e/f/1efeb61119ae49a0da8930d3070caba5a5aacf9b.png)



IMO 2003. Let an isosceles






















Commentary. The enunciation of this problem conceals vainly in its debut that the circumcircle of





Proof. Denote : the circumcircle



![$[II_{c}]$](http://latex.artofproblemsolving.com/4/d/3/4d360c2d7cf645c4c380fd8f59ddcee5827c26fd.png)




the second intersection








From the relation














Variation on the same theme. Let a fixed isosceles






for which




The circle with the diameter
where
is the
-excenter of 




This post has been edited 54 times. Last edited by Virgil Nicula, Mar 9, 2017, 8:46 AM