The sum of the first four terms of an arithmetic sequence is negative 24, and the difference between the sum of the first five terms and the sum of the first two terms is 27?

The sum of the first four terms of an arithmetic sequence is negative 24, and the difference between the sum of the first five terms and the sum of the first two terms is 27?

4*a1+6d=-24
5*a1+10d-2*a1-d=27
The solution is: A1 = - 21 d = 10
So the general formula: an = - 31 + 10N

Find the next number of rule 1.8.27, not 64!
Why 58?

58

A general formula of arithmetic sequence-1,2,5 ··

Because it is an arithmetic sequence, it can be seen from the front and back: D = 3
And because A1 = - 1
So an = a1 + (n-1) d = - 1 + (n-1) * 3 = 3n-4
So a general formula of the arithmetic sequence-1,2,5 ·· is an = 3n-4 (n ≥ 1, n ∈ n +)

Finding the general term formula and 20 term of arithmetic sequence 8.5.2

an=8-3(n-1)=-3n+11
a20=-3*20+11=-49