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HỦY DIỆT THE WORLD

03/12/2017 at 21:52
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Cho a , b , c > 0 và a + b + c = 1 .

Tìm GTNN của \(p=\sqrt{\dfrac{ab}{ab+c}}+\sqrt{\dfrac{bc}{bc+a}}+\sqrt{\dfrac{ac}{ac+b}}\)




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  • ...
    KEITA FC 8C 03/12/2017 at 22:04

    Ta có :

    \(p=\sqrt{\dfrac{ab}{\left(1-a\right)\left(1-b\right)}}+\sqrt{\dfrac{bc}{\left(1-b\right)\left(1-c\right)}}+\sqrt{\dfrac{ac}{\left(1-a\right)\left(1-c\right)}}\)

    Theo Bất Đẳng Thức Cosi ta có :

    \(2\sqrt{\dfrac{ab}{\left(1-a\right)\left(1-b\right)}}=2\sqrt{\dfrac{a}{1-b}\times\dfrac{b}{1-a}}\le\dfrac{a}{1-b}+\dfrac{b}{1-a}\)

    \(2\sqrt{\dfrac{bc}{\left(1-b\right)\left(1-c\right)}}=2\sqrt{\dfrac{b}{1-c}\times\dfrac{c}{1-b}}\le\dfrac{b}{1-c}+\dfrac{c}{1-b}\)

    \(2\sqrt{\dfrac{ac}{\left(1-a\right)\left(1-c\right)}}=2\sqrt{\dfrac{a}{1-c}\times\dfrac{c}{1-a}}\le\dfrac{a}{1-c}+\dfrac{c}{1-a}\)

    Cộng vế với vế ta được :

    \(2p\le\dfrac{a+c}{1-b}+\dfrac{b+c}{1-a}+\dfrac{a+b}{a-c}\)

    \(\Leftrightarrow2p\le\dfrac{1-b}{1-b}+\dfrac{1-a}{1-a}+\dfrac{1-c}{1-c}\)

    \(\Leftrightarrow2p\le3\)

    \(\Leftrightarrow p\le\dfrac{3}{2}\)

    Mặt khác a = b = c = \(\dfrac{1}{3}\) thì \(p=\dfrac{3}{2}\)

    Vậy maxP = \(\dfrac{3}{2}\)

    HỦY DIỆT THE WORLD selected this answer.
  • ...
    Nguyễn Thị Thanh Hiền 04/12/2017 at 07:52

    p = √one b( 1 - a ) ( 1 - b ) +√b c( 1 - b ) ( 1 - c ) +√một c( 1 - a ) ( 1 - c )

    Theo Bất Đẳng Thức Cosi ta có:

    2 √one b( 1 - a ) ( 1 - b ) =2√một1 - b ×b1 - a ≤a1 - b +b1 - a

    2 √b c( 1 - b ) ( 1 - c ) =2√b1 - c ×c1 - b ≤b1 - c +c1 - b

    2 √một c( 1 - a ) ( 1 - c ) =2√một1 - c ×c1 - a ≤a1 - c +c1 - a

    Cộng sản:

    2 p ≤ a + c1 - b +b+c1 - a +a+ba - c

    ⇔ 2 p ≤ 1 - b1 - b +1-a1 - a +1-c1-c

    ⇔2p≤3

    ⇔p≤32

    Mặt other a = b = c =  13  thì p=32

    Then maxP =  32


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