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  1. #1
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    I want check my answer

    I want check my answer

    Q 1 : If the second term = 6 and fifteenth = 25 of sequence find the first term ?
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  2. #2
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    Quote Originally Posted by r-soy View Post
    I want check my answer

    Q 1 : If the second term = 6 and fifteenth = 25 of sequence find the first term ?
    a_n = a_1 + (n-1)d


    a_2 = a_1 + d

    6 = a_1 + d


    a_{15} = a_1 + 14d

    25 = a_1 + 14d


    19 = 13d

    d = \frac{19}{13}

    a_1 = 6 - d = 6 - \frac{19}{13} = \frac{59}{13}
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  3. #3
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    e^(i*pi)'s Avatar
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    Quote Originally Posted by r-soy View Post
    I want check my answer

    Q 1 : If the second term = 6 and fifteenth = 25 of sequence find the first term ?
    No, because you haven't answered the question fully. You've been asked to find the first term a but you found the common difference d.

    Your working seems fine though. Simply sub your value of d into one of your original equations to find a.

    U_2 = a + d = 6 (eq1)

    U_{15} = a + 14d = 25 (eq2)

    Rearrange eq1:

    d = 6-a (eq3)

    sub eq3 into eq2

    a+14(6-a) = 25

    -13a = -59

    a = \frac{59}{13}
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  4. #4
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    thanks
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  5. #5
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    e^(i*pi)'s Avatar
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    Quote Originally Posted by r-soy View Post
    thanks, but why we d write

    <br />
U_2 = a + 5d = 6<br />
    Neither me nor skeeter said that? You don't write 5d because there is only d in the second term.

    It arises from the definition of an arithmetic sequence
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