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

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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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