№1 s₅: b(n) b₃=18 q=3 №2 (an) d=4 s₅₀=5500 a1=? №3 (an) a₁=10 s₁₄=1050 d-?

1. b1•q^2=18
b1•9=18
b1=18/9=2

b1•(q^5–1) 2•242
S5 = = = 242
q–1 2

2a1+49d
2. S50 =•50=(2a1+49•4)•25=
2

= (2a1+196)•25

(2a1+196)•25=5500 |:25
2a1+196=220
2a1=24
a1=12

2a1+13d
3. S14 = • 14 = (2a1+13d)•7 =
2

= (2•10+13d)•7 = (20+13d)•7

(20+13d)•7 = 1050 |:7
20+13d = 150
13d = 130
d = 10