Math
posted by Max
I swear, last question!
Calculate S(31) for the arithmetic sequence in which a(9) = 17 and the common difference is d = 2.1
46
29.2
32.7
71.3
Usually, I can figure these out but I'm stuck:(

Arora
a(n) = a(1) + (n1)d
a(9) = a(1) + (91)d
You have a(9), you have d
Use those to get a(1) from the above mentioned.
Then, plug those values into the sum formula I mentioned on the other question. 
Max
Ok, I'm so sorry, but I got so lost.
For some reason, (I think) I'm getting it wrong at determining a(1).
I don't know what the heck I'm doing, but I've gotten either 5.9, 5.9, or 11.1 for a(1) and none of them work for the other formula. I feel super dumb right now, is there any chance you could show me again how to get a(1)? Sorry 
Max
Wait I made a dumb typo. I think it's 71.3 (oh my gosh I'm dying)

Arora
a(9) = a(1) + (n1)d
a(9) = 17, d = 2.1
17 = a(1) + (91)*2.1
a(1) = 17 + 16.8
= 33.8
S(31) = (n/2)[2a(1) + (n1)d]
= (31/2)[2(33.8) + 30(2.1)]
= (31/2)(67.6  63)
= (31/2)(4.6)
= 31*2.3
= 71.3
Yes, you're correct
Respond to this Question
Similar Questions

math
what are the similarities and differences between an arithmetic sequence and a linear equation? 
Math
A sequence is formed by adding together the corresponding terms of a geometric sequence and and an arithmetic sequence.The common ratio of the geometric sequence is 2 and the common difference of the arithmetic sequence is 2.The first … 
Maths
Eric thinks of 2 sequences.One is geometric and the other arithmetic.Both sequences start with the number 3.The common ratio of the geometric sequence is the same as the common difference of the arithmetic sequence.If the 6th term … 
Maths
1..The first 2 terms of a geometric progression are the same as the first two terms of an arithmetic progression.The first term is 12 and is greater than the second term.The sum of the first 3 terms od the arithmetic progression is … 
Algebra
Determine whether each sequence is arithmetic or geometric. Find the next three terms. 14, 19, 24, 29, . . . A.geometric, 34, 39, 44 B.arithmetic, 32, 36, 41 C.arithmetic, 34, 39, 44 D.The sequence is neither geometric nor arithmetic. … 
math
The 3rd term in an arithmetic sequence is 12, the 7th term is 24, a) How many common differences are there between a_3 and a_7? 
Algebra
True or False 1. – 5, – 5, – 5, – 5, – 5, … is an arithmetic sequence. 2. In an arithmetic sequence, it is possible that the 13th term is equal to its 53rd term. 3. In an arithmetic sequence, the common difference is computed … 
Math: Arithmetic Sequence.
the 3rd of an Arithmetic sequence is 8 of the 16th term is 47. Find a subed 1 and the common difference. Construct the sequence. 
Algebra
The 15th and 21st terms of an arithmetic sequence are 67 and 97. What is the 30th term? 
Algebra
How do you tell if a sequence is arithmetic?