You don't have to find the 4th term as that woman did. But, even if you wanted to, just add 8 to the 3rd term 23,

to get the 4th term, 31. Is there any RIDICULOUS and UNNECESSARY calculation this woman has NEVER done? 

Using what was provided, the formula,  becomes: 

                                                Sum of 1st four terms or  

RELATED QUESTIONS

Find the nth partial sum of the arithmetic sequence.

an = 7n + 2; n =... (answered by MathLover1)

Find the partial sum Sn of the geometric sequence that satisfies the given conditions.
a  (answered by ikleyn)

Find the nth partial sum of the arithmetic sequence.

an = 2n − x; n =... (answered by MathLover1)

Let {an}∞n=1 be a sequence whose partial sums are {Sn}∞n=1. Suppose that a1=2  (answered by greenestamps)

Find the partial sum Sn of the geometric sequence that... (answered by stanbon)

A partial sum of the arithmetic series is given. Find the sum:  3 + 5 + 7 + . . . +... (answered by ewatrrr)

how can i find the maximum partial sum of a decreasing arithmetic... (answered by ikleyn)

A partial sum of an arithmetic sequence is given. Can you please help me find the sum?... (answered by Fombitz)

find the first 4 partial sums and nth partial sum of the sequence
An=square root n minus  (answered by Fombitz)