4.
S = b₁ + b₂ + b₃ + b₄ + b₅
b₁ = 18
b₂ = 18*1/3 = 6
b₃ = 6*1/3 = 2
b₄ = 2*1/3 = 2/3
b₅ = 2/3*1/3 = 2/9
S = 18+6+2+2/3+2/9 = 26 целых 8/9
5.
b₄ = b₂*q² => q² = b₄/b₂
q² = 64/4 = 16 => q = 4
S = b₁ + b₂ + b₃ + b₄ + b₅ + b₆ + b₇ + b₈
b₁ = 4/4 = 1
b₂ = 4
b₃ = 4*4 = 16
b₄ = 64
b₅ = 64*4 = 256
b₆ = 1024
b₇ = 4096
b₈ = 16384
S = 1 + 4 + 16 + 64 + 256 + 1024 + 4096 +16384 = 21845.
6.
b₁₈ = b₁₅*q³
q³ = b₁₈/b₁₅
q³ = 675/25 = 27 => q = 3
b₁₆ = 25*3 = 75
b₁₇ = 75*3 = 225.