FOR WHAT VALUE OF X DOES THE GEOMETRIC SERIES CONVERGE WORKSHEET

Each of the following represents an infinite geometric series. For what values of x will each series be convergent?

Problem 1 : 

5 + 5x + 5x² + 5x³ + …

Solution

Problem 2 :

1 + x/3 + x²/9 + x³/27 + …

Solution

Problem 3 :

2 + 4x + 8x² + 16x³ + …

Solution

Problem 4 :

The infinite series given by

1 + 3x + 9x² + 27x³ + …

has a sum of 4. What is the value of x?

List the first four terms of the series.

Solution

Problem 5 :

Dominique and Rita are discussing the series

-1/3 + 4/9 - 16/27 + ...

Dominique says that the sum of the series is -1/7.

Rita says that the series is divergent and has no sum.

a. Who is correct?

b. Explain your reasoning.

Solution

Answer Key

1)  The value of r = x is in the interval -1 < x < 1.

2)  So, when -3 < x < 3, the series converges.

3)  -1/2 < x < 1/2

4)  x = 1/4, 

The first four terms are

= 1 + 3/4 + 9/16 + 27/64 + …

5)  a)  Rita is correct, because the given series is divergent.

b)  Since it is divergent, we can not figure out the sum.

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