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.