Carefully observe the arrangement rules of the sequence of numbers, and choose the one you think is correct to make it conform to the arrangement rules of the original sequence of numbers: 6, 7, 8, 13, 15, 21, (), 36 A27 B28 C31

Carefully observe the arrangement rules of the sequence of numbers, and choose the one you think is correct to make it conform to the arrangement rules of the original sequence of numbers: 6, 7, 8, 13, 15, 21, (), 36 A27 B28 C31

Choose B
First number + second = fourth
Second + third = fifth
and so on
13+15=28

Find the expression of the nth term of the sequence and the sum of the first n terms: 1,1,2,3,5,8,13,21,34,55

The sequence is "Fibonacci sequence"
The general formula is: FN = 1 / radical 5 [(1 + radical 5) ^ n / 2 ^ n - (1-radical 5) ^ n / 2 ^ n]
The first N-term sum formula is: SN = 1 / radical 5 [(1 + radical 5) ^ (n + 2) / 2 ^ (n + 2)-
(1-radical 5) ^ (n + 2) / 2 ^ (n + 2)] - 1
Where ^ n is the nth power of several

Find the sum of the first n terms of sequence 5, 55555

an=(5/9)(10^n-1)
Next, sum the equal ratio sequence
an=(5/9)10^n-(5/9)
bn=an+(5/9)=(5/9)10^n
Sbn=(50*10^n-50)/81=San+(5/9)n
San=(50*10^n-50)/81-(5/9)n

In sequence 1, 1, 2, 3, 5, 8, 13, x, 34, 55 In, the value of X is ()
A. 19B. 20C. 21D. 22

According to the sequence 1, 1, 2, 3, 5, 8, 13, x, 34, 55 We can find that from the third term, each term is the sum of the first two terms, so we choose C