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Math Help - please help me: arithmetic series

  1. #1
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    please help me: arithmetic series

    The first term of an arithmetic series is 16 and the last is 60. The sum of an arithmetic series is 342. find the common difference.

    I can't work out n. Please help.
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  2. #2
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    Hello, greghunter!

    You're expected to know this formula:

    . . S_n \;=\;n\left(\frac{a_1 + a_n}{2}\right)

    \text{Sum} \;=\;n \times \text{(average of first and last terms)}


    The first term of an arithmetic series is 16 and the last is 60.
    The sum of the arithmetic series is 342. .Find the common difference.
    We have: . a_1 = 16,\;a_n = 60,\;\text{Sum} = 342

    Then: . 342 \:=\:n\left(\frac{16+60}{2}\right) \quad\Rightarrow\quad 342 \:=\:38n \quad\Rightarrow\quad\boxed{ n \:=\:9}


    ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

    If you didn't know that formula, you can still work it out.


    First term: . a_1 \:=\: 16 .[1]

    Last term: . a_n \:=\:a_1 + (n-1)d \:=\:60 \quad\Rightarrow\quad 16 + (n-1)d \:=\:60 \quad\Rightarrow\quad (n-1)d \:=\:44 .[2]

    Sum of the first n terms: . S_n \;=\;\frac{n}{2}\bigg[2a_1 + (n-1)d\bigg] \;=\;342 .[3]

    Substitute [1] and [2] into [3]: . \frac{n}{2}\bigg[2(16) + 44\bigg] \;=\;342 \quad\Rightarrow\quad n \:=\:9

    . . . . . See?

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  3. #3
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    Thanks!

    Thanks Soroban.
    I'm new to this, as you can see, and would just like to say its fantastic!

    Your working out is very clear and easy to read thankyou very much.

    Greghunter
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  4. #4
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    Quote Originally Posted by greghunter View Post
    The first term of an arithmetic series is 16 and the last is 60. The sum of an arithmetic series is 342. find the common difference.

    I can't work out n. Please help.
    Let first term=a & last term=l
    sum=n(a+l)/2
    342=n(16+60)/2
    342=n(38)
    342/38=n
    n=9
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