36
數(shù)列先減小后增大,先用后一項(xiàng)減前一項(xiàng)觀察數(shù)列減小增大的情況
( -7 ) - ( -5 ) = -2
( -8 ) - ( -7) = -1
( -6) - ( -8 ) = 2
( 1 ) - ( -6 ) = 7
9 - 1 = 8
20 - 9 = 11
( ) - 20 =
后一項(xiàng)與前一項(xiàng)的差值呈現(xiàn)遞增趨勢,用差值的后一項(xiàng)減前一項(xiàng)觀察其遞增規(guī)律
( -1 ) - ( -2 ) = 1
( 2 ) - ( -1 ) = 3
7 - 2 = 5
8 - 7 = 1
11 - 8 = 3
由上面規(guī)律可假定
- 11 = 5
解,得 = 16
( ) = 36