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algebra 3-4

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Find a1 in a geometric series for which Sn = 300, r = -3, and n = 4

  • algebra 3-4 -

    S(n) = a( r^n - 1)/(r-1)
    a( (-3)^4 - 1)/(-3-1) = 300
    a( 80 )/(-4) = 300
    -20a = 300
    a = -15

    check:
    first 4 terms would be -15 , +45, - 135, + 405
    and -15 + 45 - 135 + 405 = 300

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