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8.06 Infinite geometric series

Worksheet
Sum to infinity
1

For each of the following infinite geometric sequences:

i

Find the common ratio, r.

ii

Find the limiting sum of the series.

a

16, - 8, 4, - 2, \ldots

b
28,- 7,\dfrac{7}{4},- \dfrac{7}{16},\ldots
2

For a particular geometric sequence, t_1 = 2 and S_{\infty} = 4.

a

Find the common ratio, r of the series.

b

Write the first three terms in the geometric progression.

3

Consider the recurring decimal 0.2222 \ldots in the form \dfrac{2}{10} + \dfrac{2}{100} + \dfrac{2}{1000} + \dfrac{2}{10\,000} + \ldots Rewrite the recurring decimal as a fraction.

4

For each of the following recurring decimals:

i

Rewrite the decimal as a geometric series.

ii

Express the decimal as a fraction.

a

0.6666 \ldots

b

0.06666 \ldots

5

Consider the infinite sequence 1, x - 7, \left(x - 7\right)^{2}, \left(x - 7\right)^{3}, \ldots

For what range of values of x does the infinite series have a limiting sum?

6

Consider the recurring decimal 0.57575 \ldots in the form 0.57 + 0.0057 + \ldots Rewrite the recurring decimal as a fraction.

7

Consider the sum 0.25 + 0.0025 + 0.000025 + \text{. . .}

a

Express the sum as a decimal.

b

Hence, express the sum as a fraction.

8

The limiting sum of the infinite sequence 1, 4 x, 16 x^{2}, \ldots is 5. Solve for the value of x.

9

Consider the infinite geometric series: 6 + 2 + \dfrac{2}{3} + \dfrac{2}{9} + \ldots

a

Find the number of terms, n, that would be required to give a sum of \dfrac{177\,146}{19\,683}.

b

Find the sum of an infinite number of terms.

10

Consider the infinite geometric series: 5 + \sqrt{5} + 1 + \ldots

a

Find the exact value of the common ratio, r.

b

Find the exact value of the limiting sum.

11

Find the limiting sum of the infinite series \dfrac{1}{5} + \dfrac{3}{5^{2}} + \dfrac{1}{5^{3}} + \dfrac{3}{5^{4}} + \dfrac{1}{5^{5}} + \ldots

12

The recurring decimal 0.8888 \ldots can be expressed as a fraction when viewed as an infinite geometric series.

a

Express the first decimal place, 0.8 as an unsimplified fraction.

b

Express the second decimal place, 0.08 as an unsimplified fraction.

c

Hence, using fractions, write the first five terms of the geometric sequence representing 0.8888 \ldots

d

State the values of the first term t_1 and the common ratio t_1 of this sequence.

e

Calculate the infinite sum of the sequence as a fraction.

13

The recurring decimal 0.444444 \ldots can be expressed as a fraction when viewed as an infinite geometric series.

a

Express the first two decimal places, 0.44, as an unsimplified fraction.

b

Express the second two decimal places, 0.0044, as an unsimplified fraction.

c

Express the third two decimal places, 0.000044, as an unsimplified fraction.

d

Hence, state the values of the first term t_1 and the common ratio r of the sequence formed from these first three terms.

e

Calculate the infinite sum of the sequence as a fraction.

14

Evaluate:

a

\sum_{i=1}^{\infty} 3 \left(\dfrac{1}{4}\right)^{i - 1}

b

\sum_{i=1}^{\infty} 5 \left( - \dfrac{1}{4} \right)^{i - 1}

c

\sum_{i=1}^{\infty} \left(0.9\right)^{i}

d

\sum_{k=1}^{\infty} 9^{ - k }

e

\sum_{i=1}^{\infty} \dfrac{1}{8} \left( - \dfrac{1}{2} \right)^{i - 1}

f

\sum_{i=1}^{\infty} - \dfrac{1}{2} \left(\dfrac{5}{7}\right)^{i - 1}

Applications
15

A ball dropped from a height of 21\text{ m} will bounce back off the ground to 50\% of the height of the previous bounce (or the height from which it is dropped when considering the first bounce).

a

Write a function, y, to represent the height of the nth bounce.

b

Calculate the height of the fifth bounce. Round your answer correct to two decimal places.

16

The annual output of a coal mine decreases by twenty percent each year. The output in the first year is 274\,000\text{ m}^3.

a

Find the total amount of output in the first 8 years, to two decimal places if needed.

b

Assuming there is always enough coal in the mine for the plant's operations, find the limit to this plant's total output production, to the nearest cubic metre.

17

When a ball is dropped onto a horizontal surface from a height of 6\text{ m}, it reaches a vertical height of 50\% of the starting height on its first bounce. It continues to reach a height of 50\% of the previous height in each subsequent bounce.

a

Find the height of the first bounce.

b

Find the height of the 5th upward bounce, correct to four decimal places.

c

Calculate the total vertical distance travelled by the ball when it touches the ground after its third upward bounce.

d

If the ball continues to bounce until it finally stops, how far has it travelled vertically?

18

When a marble is rolled horizontally on a flat surface it rolls 30\text{ cm} in the first second. It then rolls 60\% of the distance travelled in the previous second, for each subsequent second.

a

Determine the distance rolled in the 2nd second.

b

Determine the total distance rolled after 3 seconds, correct to one decimal place.

c

If the marble continues to roll until it finally stops, how far has it travelled horizontally?

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Outcomes

2.2.7

understand the limiting behaviour as n→∞ of the terms t_n in a geometric sequence and its dependence on the value of the common ratio r

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