Find the sum of the first eight members of a geometrical pragresiya:-45;15;-5;....

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B1 =-45 b2 = 15 b3 =-5 b2 = b1*q q = b2/b1 = 15 / (-45) =-1/3 S(8) = b1(1-q^7)/(1-q) =-45*(1+1/2187)/(1+1/3) = (-45*2188/2187)/(4/3) = (-45*2188*3)/(2187*4) = (-45*547)/(729) =-24615/729 =-33 558/729 =-33 62/81
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