311. Some problems from USAMO and other contests.

by Virgil Nicula, Aug 17, 2011, 1:47 AM

USAMO 1979. Find for a fixed $P\in \left(\widehat{AOB}\right)$ the points $X\in (OA$ and $Y\in (OB$ so that $P\in (XY)$ and $\frac{1}{PX}+ \frac{1}{PY}$ is maximum.

Proof 1 (trigonometric). Denote $\left\{\begin{array}{ccc}
a=m(\angle POX) & ; & x=m(\angle OXP)\\\\
b=m(\angle POY) & ; & y=m(\angle OYP)\end{array}\right|$ , where $a+b+x+y=\pi$ . Apply the Sinus' theorem in the triangles:

$\left\{\begin{array}{cc}
\triangle POX\ : & \frac {PO}{PX}=\frac {\sin x}{\sin a}\\\\
\triangle POY\ : & \frac {PO}{PY}=\frac {\sin y}{\sin b}\end{array}\right|\ \bigoplus\ \implies$ $PO\cdot \left(\frac {1}{PX}+\frac {1}{PY}\right)=\frac {\sin x}{\sin a}+\frac{\sin y}{\sin b}$ . Since $PO$ and $\sin a$ , $\sin b$ are constant obtain that

$\frac{1}{PX}+ \frac{1}{PY}$ is maximum $\iff$ $\sin x\sin b+\sin y\sin a$ is maximum $\iff$ $\cos (x-b)-\cos (x+b)+\cos (y-a)-\cos (y+a)$

is maximum $\iff$ $cos (x-b)+\cos (y-a)$ is maximum because $(x+b)+(y+a)=\pi$ $\iff$ $2\cos\frac {(x+y)-(a+b)}{2}\cos\frac {x+a-y-b}{2}$

is maximum $\iff$ $2\sin(a+b)\cos\frac {x+a-y-b}{2}$ is maximum $\iff x+a=y+b=\frac {\pi}{2}$ $\iff$ $XY\perp OP$ .

Proof 2 (trigonometric). Denote $\left\{\begin{array}{c}
a=m(\angle POX)\\\
b=m(\angle POY)\\\
\phi =m(\angle OPX)\end{array}\right|$ . Apply the Sinus' theorem in the triangles:

$\left\{\begin{array}{cccc}
\triangle POX\ : & \frac {PX}{\sin a}=\frac {PO}{\sin(\phi +a)} &  \iff & \frac {1}{PX}=\frac {1}{PO}\cdot\frac {\sin (\phi +a)}{\sin a}\\\\
\triangle POY\ : & \frac {PY}{\sin b}=\frac {PO}{\sin (\phi -b)} & \iff & \frac {1}{PY}=\frac {1}{PO}\cdot\frac {\sin (\phi -b)}{\sin b}\end{array}\right|\ \bigoplus\ \implies$ $\frac {1}{PX}+$ $\frac {1}{PY}=$

$\frac {1}{PO}\cdot (\cos\phi +\cot a\cdot\sin\phi + \cot b\sin\phi -\cos\phi )=$ $\frac {1}{PO}\cdot (\cot a+\cot b)\cdot\sin\phi$ .

In conclusion, $\frac {1}{PX}+$ $\frac {1}{PY}$ is maximum $\iff\sin\phi$ is maximum $\iff\phi=90^{\circ}\iff XY\perp OP$ .

Proof 3 (synthetic). Denote $\left\{\begin{array}{ccc}
C\in (OY & ; & PC\parallel OX\\\\
Z\in (OP & ; &  CZ\parallel XY\end{array}\right|$ . Since $\left\{\begin{array}{ccc}
\triangle OZC\sim \triangle OPY & \implies & \frac {ZC}{PY}=\frac {OZ}{OP}\\\\
\triangle PZC\sim\triangle OPX & \implies & \frac {ZC}{PX}=\frac {PZ}{OP}\end{array}\right|\ \bigoplus\implies$ $\frac {1}{PX}+\frac {1}{PY}=\frac {1}{ZC}$ .

In conclusion, $\frac {1}{PX}+\frac {1}{PY}$ is maximum $\iff$ $ZC$ is minimum ($C$ is a fixed point and $Z$ is a mobile point ) $\iff CZ\perp OP\iff XY\perp OP$ .

Remark. The area $[XOY]$ is minimum $\iff$ the fixed point $P$ is the midpoint of the segment $[XY]$ . Indeed, denote $\left\{\begin{array}{cc}
C\in (OY\ ; & CP\parallel OX\\\\
D\in (OX\ ; & DP\parallel OY\end{array}\right|$

and $[OCP]=[ODP]=\sigma\ ,\ [XDP]=u\ ,\ [YCP]=v$ . Thus, the area $[XOY]$ is minimum $\iff\ u+v$ is minimum. Observe that $\frac uv=\left(\frac {PX}{PY}\right)^2=$ $\left(\frac{u+\sigma}{v+\sigma}\right)^2\implies$ $\frac uv=\left(\frac{u+\sigma}{v+\sigma}\right)^2\iff$ $u=v\ \vee\ uv=\sigma^2$ (constant). In conclusion $u+v$ is minimum $\iff u=v$ , i.e. $P$ is the midpoint of $[XY]$ .

Can easily construct the minimum position of the line $XY$. Indeed, construct $X_{\mathrm{min}}\in (OA$ so that $OX_{\mathrm{min}}=2\cdot CP$ , where $C\in (OY$ and $CP\parallel OX$ .



USA TST 2010/11. Let $ABC$ be a triangle. Its incircle touches the sidelines $BC$ , $CA$ , $AB$ at the points $D$ , $E$ , $F$ respectively and its exincircles

touch the sidelines $BC$ , $CA$ , $AB$ at the points $D'$ , $E'$ , $F'$ respectively. Prove that $\sum EF\ge \frac {2S}{R}$ and $\sum E'F'\ge s$ , where $2s=a+b+c$ .


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

$\blacktriangleright\ \sum a\cdot\cos A=$ $2R\cdot\sum \sin A\cos A=$ $R\cdot \sum\sin 2A=$ $4R\cdot\prod\sin A=$ $4R\cdot\frac {S}{2R^2}\implies$ $\boxed{\sum a\cdot\cos A=\frac {2S}{R}\ }\ \ (1)$ .

$\blacktriangleright\ \sum a\cdot (\cos B+\cos C)=$ $\sum (b\cdot\cos C+c\cdot\cos B)=$ $\sum a\implies$ $\boxed{\ \sum a\cdot (\cos B+\cos C)=2s\ }\ \ (2)$ .

$\blacktriangleright\ \sum\cos A=1-2\sin^2\frac A2+2\cos\frac {B+C}{2}\cos\frac {B-C}{2}=$ $1-2\sin\frac A2\left(\cos\frac {B+C}{2}-\cos\frac {B-C}{2}\right)=$ $1+4\prod\sin\frac A2=$

$1+4\sqrt{\prod \frac {(s-b)(s-c)}{bc}}=$ $1+\frac {4(s-a)(s-b)(s-c)}{abc}=$ $1+\frac {4sr^2}{4Rsr}\implies$ $\boxed{\ \sum\cos A=1+\frac rR\ }\ \ (3)$ . Otherwise,

remark that $2s\cdot \sum\cos A=$ $\sum a\cdot \cos A+\sum a\cdot (\cos B+\cos C)\stackrel{(1)\wedge (2)}{=}$ $\frac {2S}{R}+2s\implies$ $\sum\cos A=1+\frac rR$ .

Proof of proposed problem. Denote the projection $\mathrm{pr}_{d}([XY])$ of $[XY]$ on the line $d$ . Therefore,

$\blacksquare\ EF\ge \mathrm{pr}_{BC}([EF])=\mathrm{pr}_{BC}([EA]+[AF])=$ $(s-a)\cdot (\cos B+\cos C)$ $\implies$ $\sum EF\ge \sum (s-a)\cdot (\cos B+\cos C)=$

$2s\sum\cos A-\sum a\cdot (\cos B+\cos C)\stackrel{(3)\wedge (2)}{=}$ $2s\left(1+\frac rR\right)-2s\implies$ $\boxed{\ EF+FD+DE\ge \frac {2S}{R}\ }$ .

$\blacksquare\ E'F'\ge \mathrm{pr}_{BC}([E'F'])=a-\mathrm{pr}_{BC}(BF'+CE')=$ $a-(s-a)(\cos B+\cos C)\implies$

$\sum E'F'\ge \sum\left[a-(s-a)(\cos B+\cos C)\right]=$ $2s-2s\cdot\sum\cos A+\sum a\cdot (\cos B+\cos C)=$

$2s-2s\left(1+\frac rR\right)+2s=$ $2s\left(1-\frac rR\right)\ge 2s\left(1-\frac 12\right)$ $\implies$ $\boxed {\ E'F'+F'D'+D'E'\ge s\ }$ because $R\ge 2r$ .



USAJMO - Maryland. Prove that $a>0\ ,\ b>0\ \implies\ \frac{1}{a^2} + \frac{1}{b^2} + \frac{4}{a^2+b^2} \ge \frac{32(a^2+b^2)}{(a+b)^4} $ .

Proof. On the one hand $\frac{1}{a^2} + \frac{1}{b^2} + \frac{4}{a^2+b^2}=\frac {a^2+b^2}{a^2b^2}+\frac{4}{a^2+b^2}\ge 2\cdot\sqrt{\frac {a^2+b^2}{a^2b^2}\cdot\frac{4}{a^2+b^2}}\implies$

$\boxed{\ \frac{1}{a^2} + \frac{1}{b^2} + \frac{4}{a^2+b^2}\ge\frac {4}{ab}\ }\ (1)$ and on the other hands $\boxed {\ \frac {4}{ab}\ge\frac{32(a^2+b^2)}{(a+b)^4}\ }\ (2)\ \iff$ $(a+b)^4\ge 8ab\left(a^2+b^2\right)\iff$

$\left(a^2+2ab+b^2\right)^2\ge 8ab\left(a^2+b^2\right)\iff$ $\left(\frac ab+\frac ba+2\right)^2\ge 8\left(\frac ab+\frac ba\right)\iff$ $(y+2)^2\ge 8y$ , where $y=\frac ab+\frac ba\iff$

$(y-2)^2\ge 0$ , what is truly. In conclusion, the inequality $(2)$ is truly and so from the relations $(1)$ and $(2)$ obtain the required inequality.

Remark. $\left\{\begin{array}{c}
(a+b)^2\ge 4ab\\\\
(a+b)^2\le 2\left(a^2+b^2\right)\end{array}\right|$ . But $(a+b)^4\ge 8ab\left(a^2+b^2\right)$ . Indeed, $(a+b)^4=\left[\left(a^2+b^2\right)+2ab\right]^2=$

$\left[\left(a^2+b^2\right)^2+4a^2b^2\right]+4ab\left(a^2+b^2\right)\ge$ $2\sqrt{\left(a^2+b^2\right)^2\cdot 4a^2b^2 }+4ab\left(a^2+b^2\right)=$ $4ab\left(a^2+b^2\right)+4ab\left(a^2+b^2\right)=$ $8ab\left(a^2+b^2\right)$ .



IMO 2003. Let an isosceles $\triangle ABC$ with $AC=BC$, incentre $I$ and the circumcircle $w$ of the triangle $AIB$. Let $P\in w$ lying inside $\triangle ABC$. The lines through $P$ parallel to $CA$ and

$CB$ meet $AB$ at $D$ and $E$ respectively. The line through $P$ parallel to $AB$ meets $CA,$ $CB$ at $F,$ $G$ respectively. Prove that $DF$ and $EG$ intersect on the circumcircle of $\triangle ABC.$


Commentary. The enunciation of this problem conceals vainly in its debut that the circumcircle of $\triangle AIB$ is in fact the circle with the diameter $II_{c}\ ,$ where $I_{c}$ is the $C$- exincenter of $\triangle ABC.$

Proof. Denote : the circumcircle $w$ of $\triangle ABC$ ; the circle $\delta$ with the diameter $[II_{c}]$ ; the circumcircles $w_{a}$ , $w_{b}$ of the isosceles trapezoids $AFPL$ , $BGPD$ respectively ;

the second intersection $R$ between $\overline{FPG}$ and the circle $\delta$ ; the intersection $L\in FD\cap GE\ .$ Prove easily that $CA$ , $CB$ are the tangents from $C$ to the circle $\delta\ .$

From the relation $FA^{2}=FR\cdot FP$ obtain $\frac{FA}{AD}=\frac{GP}{PE}\ ,$ i.e. the quadrilaterals $FADP$ , $GPEB$ are similarly as $FADP\sim GPEB\ .$ Thus,

$\begin{array}{cc}1\blacktriangleright & \begin{array}{c}\widehat{AEL}\equiv\widehat{GEB}\equiv\widehat{FDP}\equiv\widehat{AFD}\equiv\widehat{AFL}\Longrightarrow\widehat{AEL}\equiv\widehat{AFL}\Longrightarrow L\in w_{a}\\\\ \widehat{BDL}\equiv\widehat{FDA}\equiv\widehat{GEP}\equiv\widehat{BGE}\equiv\widehat{BGL}\Longrightarrow\widehat{BDL}\equiv\widehat{BGL}\Longrightarrow L\in w_{b}\end{array}\Longrightarrow\boxed{\ \widehat{DLE}\equiv\widehat{ABC}\ }\\\\ 2\blacktriangleright & \begin{array}{c}\widehat{ALD}\equiv\widehat{ALF}\equiv\widehat{APF}\\\\ \widehat{BLE}\equiv\widehat{BLG}\equiv\widehat{BPG}\end{array}\Longrightarrow \widehat{ALD}+\widehat{BLE}\equiv\widehat{APF}+\widehat{BPG}\equiv\widehat{AI_{a}B}\equiv\widehat{ABC}\Longrightarrow \boxed{\ \widehat{ALD}+\widehat{BLE}\equiv\widehat{ABC}\ }\end{array}$ $\Longrightarrow$

$\widehat{ALB}\equiv\widehat{ALD}+\widehat{BLE}+\widehat{DLE}=$ $2\cdot\widehat{ABC}=$ $180^{\circ}-\widehat{ACB}$ $\Longrightarrow$ $\boxed{\ \widehat{ALB}+\widehat{ACB}=180^{\circ}\ }$ $\Longrightarrow$ $L\in w\ .$

Variation on the same theme. Let a fixed isosceles $\triangle ABC,$ where $(AB=AC)$ and let mobile $M\in [AB,$ $N\in [AC$ so that $MN\parallel BC$ and exists $P\in (MN)$

for which $PM\cdot PN=BM^{2}.$ Construct $\{S,T\}\subset (BC)$ for which $PS\parallel BM\ ,\ PT\parallel NC\ .$ Find the locus of the intersection $L\in MS\cap NT.$
Answer.
This post has been edited 54 times. Last edited by Virgil Nicula, Mar 9, 2017, 8:46 AM

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