Math

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:(

  1. 0
  2. 0
  3. 12
asked by Max
  1. a(n) = a(1) + (n-1)d
    a(9) = a(1) + (9-1)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.

    1. 0
    2. 0
    posted by Arora
  2. 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

    1. 0
    2. 0
    posted by Max
  3. Wait-- I made a dumb typo. I think it's 71.3 (oh my gosh I'm dying)

    1. 0
    2. 0
    posted by Max
  4. a(9) = a(1) + (n-1)d
    a(9) = 17, d = -2.1

    17 = a(1) + (9-1)*-2.1
    a(1) = 17 + 16.8
    = 33.8

    S(31) = (n/2)[2a(1) + (n-1)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

    1. 0
    2. 0
    posted by Arora

Respond to this Question

First Name

Your Response

Similar Questions

  1. 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
  2. Calculus

    Wouldn't this sequence be neither geometric and neither arithmetic? 0.0005, 0.005, 0.05, 0.5 I can'f find a pattern. And also this sequence, it doesnt look arithmetic or geometric 1/5, 1/4, 1/3, 1/2, 1,... And also this other
  3. math

    Determine whether each sequence is arithmetic or geometric. Find the next three terms. 1. 14, 19, 24, 29, . . . (1 point) geometric, 34, 39, 44 arithmetic, 32, 36, 41 arithmetic, 34, 39, 44 *** The sequence is neither geometric
  4. Math Help!!!

    determine whether each sequence is arithmetic or geometric. find the next three terms. 1. 14,19,24,29.... geometric, 34,39,44 arithmetic,32,36,41 arithmetic 34,39,44**** the sequence is nether geometric nor arithmetic 2.
  5. math

    term given by x+2,3x-1,and 4x-1 form an arithmetic sequence. find the value of x. i didn't get it how to start this question and how it forms arithmetic sequence?
  6. algebra

    can you check my answers please? What are the first three terms of the sequence: a1 = 3 and an = 2(an1)2? 2, 8, 18 3, 18, 648 --3, 32, 50 2, 6, 12, 24 What is the 14th term in the sequence: an = 4n + 13? -43 43 __> -69 69 What is
  7. Can someone help me?!

    The 1st, 5th and 13th terms of an arithmetic sequence are the first three terms of a geometric sequence with a common ratio 2. If the 21st term of the arithmetic sequence is 72, calculate the sum of the first 10 terms of the
  8. math

    The 1st,5th,13th term of an arithmetic sequence are the first 3 terms of geometric sequence with a common ratio of 2. If the 21st term of the arithmetic sequence is 72, calculate the sum of the first 10 terms of the geometric
  9. math

    1. Use the given arithmetic sequence to answer the following problems. Arithmetic sequence: 10, 15, 20, 25, 30, …. a. Use the formula to find the 75th Term of the given Arithmetic Sequence I. Formula: II. Work: b. Use the
  10. 7th Grade Math for Steve.. or Ms. Sue

    Determine whether each sequence is arithmetic or geometric. Find the next three terms. 1. 14, 19, 24, 29, . . . (1 point) geometric, 34, 39, 44 arithmetic, 32, 36, 41 arithmetic, 34, 39, 44 ** The sequence is neither geometric nor

More Similar Questions