Answer:
a) 1456/243
b) 170
Step-by-step explanation:
The sum of n terms of a geometric sequence with first term a1 and common ratio r is given by ...
Sn = a1·(r^n -1)/(r -1)
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a)
The first term is 4; the common ratio is 1/3. You want the sum of 6 terms:
S6 = 4·((1/3)^6 -1)/(1/3 -1) = 4(-728/729)/(-2/3) = 1456/243
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b)
The first term is -2; the common ratio is -2. You want the sum of 8 terms:
S8 = -2((-2)^8 -1)/(-2 -1) = -2(255/-3) = 170
