Use the formula of a geometric sequence and find the sum of the first five terms. a1 = -3.449 and r = 1.727

Question

a. 

28.68

b. 

-37.46

c. 

-68.14

d. 

19.76

Answer 1

S_n = a₁(1-rⁿ)/(1-r) where r≠1

S₅ = -3.449(1-1.727⁵)/(1-1.1727) ≈ -68.14

Hope this helped!

Answer 2

odd term will be negative so eliminate a and d

signs are all negative, magnitude ballooning

a1 is negative

so is a3, a5 = the 5th term

s5 is also negative, as a1+a3+a5 is greater than a2+a3

correct answer has to be b or c

try a simpler version let a1 =-3, r=2

-3,-6,-12,-24,-48 sum = -93 D is closest

had sequence alternated in sign and r=about -2

then sequence is -3, 6, -12, 24, -48 and sum of 1st five = -63+30 = -33

b is a good guess

but not guaranteed