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
а1 = 2/3
а2 =
а3 =
а4 =
а5 = 54
54 = 2/3* q^4
q = √ √ 54/2/3 = √√ 81 = 3
а2 = 2/3* 3 = 2
а3 = 2*3 = 6
а4 = 6 * 3 = 18
B⁵=-6 b⁷=-54 b⁷/b⁵=q²=54/6=9
q=3 b⁵=b1*q⁴ =-6 b1=-6/81=-2/27
S6=-2/27(1-3⁶)/(1-3)=-2/27*729/2=-729/27=-27
q=-3
b⁵=b1*q⁴ =-6 b1=-6/81=-2/27
S6=-2/27(1-3⁶)/(1+3)=-2/27*729/4=-729/27=-27/2=-13.5