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Sequences and series, don't understand how to do this watch

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    New question! How would I do

    The sum of the first ten terms of an AP is 95, and the sum of the first 20 terms of the same AP is 290. Calculate the first term and the common difference.


    I've done

    S_{10}=5[2a+9d]=95
    S_{20}=10[2a+19d]=290

    Thought about simultaneous equation, tried it but it didn't really come to much..



    In an arithmetic series, the 3rd term is 4 and the 8th term is 49. Find the 1st term, the common difference and the sum of the first 10 terms.

    I understand what to do when I know what the first term is.

    How do I found that out in this question?

    Thankyou.
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    u_1 = a

    u_2 = a + d

    u_3 = a + 2d

    so a + 2d = 4

    can you see how to continue?
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    You will get two equations as above, and you have two unknowns :-)
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    (Original post by Pheylan)
    u_1 = a

    u_2 = a + d

    u_3 = a + 2d

    so a + 2d = 4

    can you see how to continue?
    not really

    I have a+2d=4 and a+7d=49...
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    (Original post by Kash:))
    a+2d=4 and a+7d=49...
    :yes: solve them simultaneously
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    (Original post by Pheylan)
    :yes: solve them simultaneously
    :awesome: thankyou
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    (Original post by Kash:))
    not really

    I have a+2d=4 and a+7d=49...
    :lol:

    See post 6.
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    New question! How would I do

    The sum of the first ten terms of an AP is 95, and the sum of the first 20 terms of the same AP is 290. Calculate the first term and the common difference.


    I've done

    S_{10}=5[2a+9d]=95
    S_{20}=10[2a+19d]=290

    Thought about simultaneous equation, tried it but it didn't really come to much..
 
 
 
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Updated: February 4, 2010

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