Question

(an) - геометрическая прогрессия. найдите S5, если a1= 18, q= - 12

Answer 1

S5 = a1*(q^5-1)/(q-1) = 18*((-1/2)^5-1)/(-1/2-1) = 18*(-1/32)-1)/(-3/2 = 18*(-33/32)*(-2/3) = 12 3/8 = 12,375