Question

найдите сумму первых восьми членов геометрической прагрэсии : -45;15;-5; ... .

Answer 1

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