Question

Find the sum of the infinite geometric series -27, -9, -3, … The ratio is /3 and u1 is -27

Answer 1

Answer: C) -81/2

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Work Shown:

a = -27 = first term

r = 1/3 = common ratio, note how this is between -1 and 1

We start with -27 and multiply by 1/3 each time to get the next term

S = infinite sum

S = a/(1-r), which only works because -1 < r < 1 is true

S = -27/(1-1/3)

S = -27/(2/3)

S = (-27/1) divided by (2/3)

S = (-27/1) times (3/2)

S = (-27*3)/(1*2)

S = -81/2

As you generate and add up the terms of the sequence, the infinite sum slowly starts to approach -81/2 = -40.5; we'll never actually achieve this sum exactly. Think of it as approaching an asymptote.