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1 . 1 ( )∆ = ∆ = = = +� �MAN PAN S S AD PN a x y

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PAN

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2

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MN =MC +CN � x y+ = −a x + −a y0,25

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2 ( )

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x + y + xy a= + x − ax a+ + y − ay� xy a x y+ + =aa

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ax= −y x a� +Th  vào (*) ta đế ược 1 (

2

)= + −a axS a x+

MAN

2x aĐ t ặ

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� + � + −( ) '( ) . 2a x a a x ax a= �� + ��� = +f x f x.'( ) 0 ( 2 1)f x = � x= − a

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2 2 ( )x a x a(( 2 1) )

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( 2 1)f = f a = af − a =a −(0) ( )2, f x = a�

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min ( )

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f x =a −

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max ( )

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max 3V = a

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M C N D

S AMN

6V y  khi ậMB ND a= = −V = − a3( 2 1)( 2 1)min

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3         khi 

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+ + + + + + + + + +1 1 1 5( )a ab b bc c ca a b cV 1,00+ + + + + +

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3 3 3a ab c b bc a c ca b, 0∀x y>

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x 2x y xy x xy y x y+�۳ ۳− − y ta có + + = + + + + − + +

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a ab a ab a ab a ab ca b1 ( 1) 2( 1) ( 3 )= + − − + + + − −

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2

2

2

2

2

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a c ab a b c a c +2 2( )+ +3 3a ab c

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a + b + c a +a +a +a +a + + + + +b b b c c5 3 2 (10)( )= =2 20( )

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5 3 2a a a a a b b b c c+ + + + + + + + + = a+ b+ c2 5 2 5Tương t , c ng l i ta đự ộ ạ ược1a b c= = = 3Đ ng th c x y ra ẳ ứ ả

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B

x

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M

y

C

D

P

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