Найдите сумму первых пяти членов прогресии 2/3, 1/2, 3/8.

2/3, 1/2, 3/8 ... S₅-?

q = (1/2) / (2/3) = 3/4

a₁=2/3

S₅ = (2/3) ((1-3/4) ⁵) / (1-3/4) = (2/3) ((1-243/1024) - 1) / (1/4) =

= (2/3) (781/1024) / (1/4) = 2*781*4 / (3*1024) = 781/384=2¹³/₃₈₄.