Compare Expressions (Hard)

Algebra Level 3

Give:

Compare a+b+c with abc

\frac { 1 }{ 2 } (a+b+c)=abc a+b+c > abc No answers true a+b+c < abc

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1 solution

We can use Viète to solve this problem : a = 3 + 2 3 1 1 2 3 3 5 = ( 3 1 + 1 ) 2 ( 3 1 2 3 ) 2 ( U s e V i e ˋ t e ) = . . . = 1 + 2 3 b = 3 + 2 3 1 1 + 2 3 3 5 = ( 3 1 + 1 ) 2 ( 3 1 + 2 3 ) 2 = . . . = 1 2 3 c = 6 3 + 2 9 3 3 4 3 2 3 3 = ( 3 + 3 3 ) 2 ( 3 3 1 ) 2 = . . . = 3 + 1 T h e n : a + b + c = ( 1 + 2 3 ) + ( 1 2 3 ) + ( 3 + 1 ) = 3 + 3 > 2 + 3 > 2 > 0 a b c = ( 1 + 2 3 ) ( 1 2 3 ) ( 3 + 1 ) = 2 S o : a + b + c > a b c a=\sqrt { \sqrt { 3 } +2\sqrt { \sqrt { 3 } -1 } } -\sqrt { 1-2\sqrt { 3\sqrt { 3 } -5 } } \\ \quad =\sqrt { { (\sqrt { \sqrt { 3 } -1 } +1) }^{ 2 } } -\sqrt { { (\sqrt { \sqrt { 3 } -1 } -\sqrt { 2-\sqrt { 3 } } ) }^{ 2 } } (\quad Use\quad Viète)\\ \quad =...=\quad 1+\sqrt { 2-\sqrt { 3 } } \\ b=\sqrt { \sqrt { 3 } +2\sqrt { \sqrt { 3 } -1 } } -\sqrt { 1+2\sqrt { 3\sqrt { 3 } -5 } } \\ \quad =\sqrt { { (\sqrt { \sqrt { 3 } -1 } +1) }^{ 2 } } -\sqrt { { (\sqrt { \sqrt { 3 } -1 } +\sqrt { 2-\sqrt { 3 } } ) }^{ 2 } } \\ \quad =...=\quad 1-\sqrt { 2-\sqrt { 3 } } \\ c=\sqrt { 6-\sqrt { 3 } +2\sqrt { 9-3\sqrt { 3 } } } -\sqrt { 4-\sqrt { 3 } -2\sqrt { 3-\sqrt { 3 } } } \\ \quad =\sqrt { { (\sqrt { 3 } +\sqrt { 3-\sqrt { 3 } } ) }^{ 2 } } -\sqrt { { (\sqrt { 3-\sqrt { 3 } } -1) }^{ 2 } } \\ \quad =...=\sqrt { 3 } +1\\ Then:\quad a+b+c=(1+\sqrt { 2-\sqrt { 3 } } )+(1-\sqrt { 2-\sqrt { 3 } } )+(\sqrt { 3 } +1)=3+\sqrt { 3 } >2+\sqrt { 3 } >2>0\\ \quad \quad \quad \quad abc=(1+\sqrt { 2-\sqrt { 3 } } )(1-\sqrt { 2-\sqrt { 3 } } )(\sqrt { 3 } +1)\quad =2\\ So:\quad a+b+c>abc

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