mathematics

1}
the first term of an AP is 2 and the common difference is 3.Find the sum of the first 11 terms. 2}The 2nd and 5th terms of a GP are -6 and 48 respectively.Find the sum of the first four terms

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  1. using our basic formulas
    S(11) = (11/2)(4 + 10(3))
    = 187

    t2 = ar = -6
    t5 = ar^4 = 48
    divide them:
    r^3 = 48/-6 = -8
    r = -2

    in ar = -6
    a = 3

    sum(4) = a(r^4 - 1)/(r-1)
    = 3((-2)^4 - 1)/(-2-1)
    = 3(15)/-3
    = -15

    check:
    first 4 terms are:
    3 , -6, 12, -24
    add them to get -15

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  2. 15

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