deva dan Toan HSG 9. VPhuc nam 2009-2010

    !" #$   %&'(')*+,  ( ) ( )            y x x x y x xy y  = + +   − + = + −   %&'('*+,   ! n "#$%& '( a )  a n≤ *+,*- %&/'(/'*+,  .,  *    /  ABC  0  +  12  3  4 O 5-  6 6AD BE CF ')  7*  12  *, ( ) 6 6D BC E CA F AB∈ ∈ ∈ -829 EF : BC ; 6G 129 AG :';123 4 5O ;1< M - - .=>71< 6 6 6A M E F ?> 0123- - / N ')1<; BC $) H ')@A* ABC -.=> GH AN ⊥ %&0'(')*+,  .=>  ( )  B B     4 54 54 5  a b c abc a b b c c a a b b c c a abc + + + + + + ≥ + + + + + + $%/ 6 6 Ca b c > %&)'('*+, D&E$E1$FG*7H#% C C× 4C!36C051I0  !HE$IJC*,,7HK*),,*E0;,L*E 01MG*7')* ?*-.=>"1I#N'O- P+  !" #$% 12342567+45 888888888888888888889:;<=>45 8888888  ?@A B C4 <;4 %&'(')*+, ' D+=&4E2FG45:HI +,- Q+';=*G*$R!; ( ) ( )        Cy x y x x− + + + − = .,1A')7S*6T 6y x ')*-." ( ) ( )    U      Vx x x x∆ = + − + − = C- W1"61I  6  y x y x= − = + C- X Y+ y x= − 6*$),=61I C6 6 x x x= = − = − C- • Q% Cx =   y x= − = • Q% x = −   y x= − = • Q% x = −   Vy x= − = C- X Y+  y x= + 6*$),=61I C6 6 Vx x x= = − = C- • Q% Cx =    y x= + = • Q% x = −    y x= + = − • Q% Vx =    VVy x= + = C- QS6G*') ( ) ( ) ( ) ( ) ( ) ( ) Z CZ 6 Z 6 Z  6 ZV 6 VZVVx y = − − − − C- %&'('*+, D+=&4E2FG45:HI +,- / a ')'('%)  -a n≤ [ ( )  n a< + C- Y+ Na ≥  6 6a a a− − ')%'(G* -n 8<\>6) ?*1E 06  ( ) ( )   ]a a a n− − -^* ( ) ( ) ( ) ( ) ( )  B      N   C N   Ca a a n a a a a a a a− − ≤ < + ⇒ − + − < ⇒ − + − < -QE'\4!, Na ≥ 5- C- _,1" a = ,L Ba = ,L a = C- X Y+ a =  { }    B 66B6666N6n n≤ < ⇒ ∈ C- X Y+ Ba =  { }   B  V66666n n≤ < ⇒ ∈ 4!, 6B]n 5 C- X Y+ a =  { }    N BC6n n≤ < ⇒ ∈ 4!, 6B6]n 5 C- QS ! n O')66B6666N66V666666BC6 C- %&/'(/'*+, D+=&4E2FG45:HI +,- &'() 6 6 6A M E F  *('$+', ') 5J4KL2.,=`a._6b')*,1<G*`a$)._-=`a._0+ H$)MH - -PA PB PC PD=  X c!dSef ,= AMBC 0+6*1I GM GA GB GC × = ×  /0 123/45 )% C- X c!d,= BFEC 0+6*1I GB GC GF GE× = × C- X ^* GF GE GM GA× = × C- X _,1"6= AMFE 0+- C- 670 N '/ 8 BC 4 H '9 :  ABC &'( ') ;+<25 =>'6?6 GH AN⊥ X g,H+JO6$)=`hPi0+* M > 12312H# AH 6 !,1" HM MA⊥ - C- X * MH :';123 4 5O ; K 6H1"!, VCAMK∠ = o   AK ')12H#G* 4 5O - C- X W1"* 6KC CA KB BA⊥ ⊥ -^* ]] 6 ]]KC BH KB CH 6!,1" BHCK ')7)- ^* KH 1J* N C- X [1" 6 6M H N 9)- C- X ,* GAN "*12*, 6AD NM :*; 6H   H ')@AG**  GAN -^* GH AN⊥ C- %&0'(')*+, ' N D K M G F E H O B C A D+=&4E2FG45:HI +,- a19=O=1$% ( )  B B     4 54 54 5  a b b c c a a b c abc a b b c c a abc   + + + + + + ≥ + + +  ÷ + + +   C- .=    4 54 54 5 4 5 4 5 4 5 a b b c c a c a b a b c b c a abc+ + + = + + + + + + C- c!d719=.*Xa*,HX^j*k ( ) ( )    B  B  B     4 5 4 5 4 5       - - -  -  c a b a b c b c a abc a b b c c a abc c a b a b c b c a abc a b b c c a abc c a b abc   + + + + + + + + + ≥  ÷ + + +     ≥ + + + + + +  ÷  ÷ + + +   = + + + _lmne*H$)MH  4 5 4 5 4 5  -c a b a b c b c a abc abc a b c+ = + = + = ⇔ = = C- C- %&)'('*+, D+=&4E2FG45:HI +,- X  &$E,6H#%  × M"HEJ*+,6o$S6" HEJ*+,B C- X pH#77q$E6H#%  × 6"R*+,6"R *+,B-_,1"6"#C3';HE*+,6oHE* +,B-Q$S6r')0,66N- X W1"6g, '\_'g6"0e#N'O- C- 4;0 1    )% ;+<25 =>'@?6 .  6 6AD BE CF ')  7*  12  *, ( ) 6 6D BC E CA F AB∈ ∈ ∈ -82 9  EF : BC ; 6G 12 9  AG :';123 4 5O ;1< M - - .=>71< 6. ?*-.=>"1I#N'O- P+  !" #$% 12342567+45 88888888888888888888 9 :;<=>45 8888888  ?@A B C4. _,1"6= AMFE 0+- C- 670 N '/ 8 BC 4 H &apos ;9 :  ABC &'( ') ;+<25 =>'6?6 GH