( x + 2 ) + ( x + 5 ) + ( x + 7 ) + ( x + 12 ) +....+ ( x + 81 ) = 447.
Ta có dãy fibonaxi:
Dãy đầy đủ là:
( x + 2 ) + ( x + 5 ) + ( x + 7 ) + ( x + 12 ) + ( x + 19 ) + ( x + 31 ) + ( x + 50 ) + ( x + 81 ) = 447
=> x + 2 + x + 5 + x + 7 + x + 12 + x + 19 + x + 31 + x + 50 + x + 81 = 447
=> ( x + x + x + x + x + x + x + x ) + ( 2 + 5 + 7 + 12 + 19 + 31 + 50 + 81 ) = 447
=> 8x + 207 = 447
=> x = ( 447 - 207 ) : 8
=> x = 30.