    !"#" $%&'() Thi gian lm bi: 120 pht, không k thi gian giao đê *+,  /&'()0&'1%-2%34"56&"7 8&'(-94)&:;%- <=5>?(@47   !" ( ) ( )  # $  #  x y y x  = −   = −   %& '()*+ #  , -   a a a P a a − + = +  <="5"?(@47 ./ !"  #01 2$234  !"2$5$ 6"7 7!( !"2$8 9 $ :  ;<   $  $ $ # #x x+ + + = <=>5?(@47 =/ 7 >?@ ABC3D5EFG@H?A??CI!J) 3K?AE6L BBM3- L25@ 5(L  NE6OBP@ !/ QR3S I?BPQ35TE <=A5>?(@47 ./"B)@ AC.U8GVKW IF3G(X!RKC.2F 57CBY3(X!RK.U2Y57.//FAY- 1 EZL [/ CU\AFBAY3]3^K_B`E .* * 7ACF`3* 7G?E 'a3 /(F`BY_E.* AaB)@  8BQFYE b7B!O(FB(Y( 7AFY8VO3QcE <=B5?(@47 .* + # 0 # ≥20BQ'I≥1EdVe 5?J)!RI*  cf * # # # # # # $ $ $ $ $ $ $a b b c c a + + ≤ + + + + + + BQ'II37V ;<EE$ a? a'RO+EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEg7/V+EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE .>5O 7$+EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE.>5O 7+EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE <=5>?(@47  Z3L f  \hK(2:1B\hK(21:  !" ( ) ( )  # $  #  x y y x  = −   = −   6#B/2^E2##⇒# 6#B/2$^E##⇒# P  !" # # x y =   =  2#:# %& '()*+ #  , -   a a a P a a − + = +  ( ) , -   a a a a a a − + = +  ( ) ( )     a a a a a a + = = + <="5"?(@47 ./ !"  #01 2$234  !"2$5$ Q$8 !"+  #0$1 ∆  2#  E$E$-i1⇒ !"8 9 $ # -   b x a − − ∆ − = = B  # -   b x a − + ∆ + = = PBQ$"P  !" # - # - :   S   − +   =       Z( !"8 9 $ :  6"∆i1⇔,i1⇒ ,  m <  =8Ij/j8+ $  $  # b x x a c x x m a −  + = =     × = =   FS57+ ( ) ( ) ( ) ( )       $  $  $  $ $ # # $ $  $ $ kx x x x x x+ + + = ⇔ + + + + + + = ( ) ( ) ( ) ( ) ( )      $  $  $  $   $ $ - -  - ,   Dx x x x x x x x m m⇒ + + = − + = − + + = − + = + ( ) ( )        $  $  $  $ $ D  $l lx x m x x x x m m⇒ + + = + ⇒ + + + = + + ( )     $  $  $   $ $l lx x x x x x m m⇒ + + − + = + +   ,  $ $l l $D - #m m m m m m⇒ + − + = + + ⇒ = − ⇒ = −  P#" !"8 9 $ :  ;<T5   $  $ $ # #x x+ + + = <=>5?(@47 '25T3BP@3&QR3S E2T5iE =8 P@3&)@Vm 025TIBP@3& ^Vm 25T 6L @)@Vm  D x + 2IL @ ^Vm  D x − 2 6L BBM25@ OL  N3- H/R8 !" D D -  x x + = + − 2n  !"2n⇔  - ,l D1 1x x− − = 2 o k1 o -∆ = ⇒ ∆ = ⇒ $    - − < 23/K:  1 PBP@3&QR3S 315TE <=A5>?(@47 <=B5?(@47 .* + # 0 # ≥20BQ'I≥1 6]* + # 0 # 20≥1 68 # 0 # 20202  0  20202  0  0  202  U/I≥1⇒0≥1B2  ≥1⇒202  ≥1P # 0 # ≥20BQ'I≥1 dVe 5?J)!RI* cf * # # # # # # $ $ $ $ $ $ $a b b c c a + + ≤ + + + + + + BQ'II 37V ;<EE$ 68 # 0 # ≥205?^BQEE$ ⇒ # 0 # 0$≥200200⇒ ( ) # # $ $ abc c a b ab a b c a b c ≤ = + + + + + + 6 p # # $ $ a b c a b c ≤ + + + + B # # $ $ b c a a b c ≤ + + + + U/8 # # # # # # $ $ $ $ $ $ $ c b a a b c a b b c c a a b c a b c a b c a b c + + + + ≤ + + = = + + + + + + + + + + + + + +