B1 = ¼; q = 1/2; bn = 1/512
Sn - ?
bn = b1 * q^(n-1) => q^(n-1) = bn/b1
q^(n-1) = 1/128
1/2^(n-1) = (1/2)^7
n - 1 = 7
n = 8
Sn = b1 * (qⁿ - 1)/(q - 1) = 1/4 * (1/256 - 1)/(1/2 - 1) = 1/4 * 2*255/256 = 255/512
Ответ: 255/512
Lg(x +1)/x > 0 (x +1)/x > 0
------------------ -1-------------------0------------ x∉(-1;0)
+ - +
(x +1)/x > 1 1+1/x-1 > 0 → 1/x> 0 x> 0