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Posted by on Saturday, March 23, 2013 at 7:38pm.

1)The sum of 6 terms of an arithmetic series is 45, the sum of 12 terms is -8. Find the first term and the common difference?

2)If in a geometric sequence the sum of the second and the third is 20 and the sum of the fourth and fifth is 320, find the common ratio and the first term. Assume that the common ratio is possitive.

  • Math(arithmetic & Geometric Series) - , Saturday, March 23, 2013 at 9:15pm

    You have to know the formula for the sum of n terms of an AS

    Sum(6) = 3(2a + 5d)
    6a + 15d = 45 --- #1

    sum(12) = 6(2a + 11d ) = -8
    12a + 66d = -8
    6a + 33d = -4 ---#2

    subtract them
    18d = -49
    d = -49/18

    back in #1
    6a + 15(-49/18) = 45
    6a = 45 + 735/18
    6a = 515/18
    a = 515/36

    expected nicer numbers....

    #2:

    ar + ar^2 = 20 ---> ar(1 + r) = 20 ----#1
    ar^3 + ar^4 = 320 ---> ar^3(1 + r) = 320 ---#2

    divide #2 by #1
    r^2 = 16
    r = 4 , you said r is positive

    in #1
    4a(5) = 20
    a = 1

    a = 1 , r = 4

    check:
    terms would be
    1 4 16 64 256
    and 4+16 = 20
    and 64 + 256 = 320

  • Math(arithmetic & Geometric Series) - , Wednesday, February 10, 2016 at 2:13am

    the common different between p and q

  • Math(arithmetic & Geometric Series) - , Wednesday, November 16, 2016 at 5:16am

    The third term of geometric sequence is 108 and the 6term is -32 find the common ratio ,first term and sum of first term

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