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47 .   Let   cos - 1 ( x ) + cos - 1 ( 2 x ) + cos - 1 ( 3 x ) = π .   If   x   satisfies   the   cubic   ax 3 + bx 2 + cx - 1 = 0 ,   then   a + b + c   has   the   value   equal   to   : ( a )   24                                   ( b )   25                                         ( c )   26                                       ( d )   27

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