36
数列先减小后增大,先用后一项减前一项观察数列减小增大的情况
( -7 ) - ( -5 ) = -2
( -8 ) - ( -7) = -1
( -6) - ( -8 ) = 2
( 1 ) - ( -6 ) = 7
9 - 1 = 8
20 - 9 = 11
( ) - 20 =
后一项与前一项的差值呈现递增趋势,用差值的后一项减前一项观察其递增规律
( -1 ) - ( -2 ) = 1
( 2 ) - ( -1 ) = 3
7 - 2 = 5
8 - 7 = 1
11 - 8 = 3
由上面规律可假定
- 11 = 5
解,得 = 16
( ) = 36