Question 1181059

Use the partial sum formula to find the partial sum of the given arithmetic sequence. ( Sn =  n/2 ( a1 + an)  )

Find the sum of the first four terms of the arithmetic sequence: 7, 15, 23, . . . .

a. 60
b. 45
c. 152
d. 76
<pre>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, {{{matrix(1,3, S[n], "=", (n/2)(2a[1] + (n - 1)d))}}} becomes: {{{highlight_green(matrix(3,3, S[4], "=", (4/2)(2(7) + (4 - 1)8), S[4], "=", 2(14 + 3(8)), S[4], "=", 2(14 + 24)))}}}
                                                Sum of 1st four terms or {{{highlight_green(matrix(1,5, S[4], "=", 2(38), "=", 76))}}}