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    The question is the sum of the first 30 terms of an arithmetic sequence with the first term 7 is equal to 450, find the thirtieth term. I have tyres subbing into all three formulas but I don't get it
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    formula of sum = n/2(a+l)

    where a = first term, l = last term aka 30th term in your case
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    (Original post by Bealzibub)
    formula of sum = n/2(a+l)

    where a = first term, l = last term aka 30th term in your case
    Dosent work?
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    (Original post by SunDun111)
    Dosent work?
    Yes it does. 450 = 30/2 (7+L)

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    (Original post by samb1234)
    Yes it does. 450 = 30/2 (7+L)

    Posted from TSR Mobile
    Oh yeah sorry my bad,
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    (Original post by samb1234)
    Yes it does. 450 = 30/2 (7+L)

    Posted from TSR Mobile
    One more question, these are the only questions believe it or not I can't do, in an arthimitic sequence the ninth term is 7 and the twenty ninth is equal to twice the fifth term. Find a and d. Straight away I know it's simultaneous and my first equation is 7 = a + 8d but how do I get my second
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    (Original post by SunDun111)
    One more question, these are the only questions believe it or not I can't do, in an arthimitic sequence the ninth term is 7 and the twenty ninth is equal to twice the fifth term. Find a and d. Straight away I know it's simultaneous and my first equation is 7 = a + 8d but how do I get my second

    twenty ninth term = 2 * 5th term

    fifth term = a + 4d

    twenty ninth term = 2(a+4d)

    2(a+4d) = a + 28d
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    (Original post by Bealzibub)
    twenty ninth term = 2 * 5th term

    fifth term = a + 4d

    twenty ninth term = 2(a+4d)

    2(a+4d) = a + 28d
    Don't understand where does 28 come from urghh my bad not good with sequences big weakness..
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    (Original post by SunDun111)
    Don't understand where does 28 come from urghh my bad not good with sequences big weakness..
    28 = 29 - 1

    You were doing OK when you set up the first equation because you saw where the numbers came from. You do exactly the same thing when you write down an expression for the 29th term
 
 
 
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