1. Objectives Find sums of infinite geometric series.
Use mathematical induction to prove statements.

2. In Lesson 9-4, you found partial sums of geometric series
In Lesson 9-4, you found partial sums of geometric series. You can also find the sums of some infinite geometric series. An infinite geometric series has infinitely many terms. Consider the two infinite geometric series below.

3. Notice that the series Sn has a common ratio of and the partial sums get closer and closer to 1 as n increases. When |r|< 1 and the partial sum approaches a fixed number, the series is said to converge. The number that the partial sums approach, as n increases, is called a limit.

4. For the series Rn, the opposite applies
For the series Rn, the opposite applies. Its common ratio is 2, and its partial sums increase toward infinity. When |r| ≥ 1 and the partial sum does not approach a fixed number, the series is said to diverge.

5. Example 1: Finding Convergent or Divergent Series
Determine whether each geometric series converges or diverges. A B
The series converges and has a sum.
The series diverges and
does not have a sum.

6. Check It Out! Example 1
Determine whether each geometric series converges or diverges. A.
B …
The series diverges and
does not have a sum.
The series converges and has a sum.

8. Example 2A: Find the Sums of Infinite Geometric Series
Find the sum of the infinite geometric series, if it exists.
1 – –
r = –0.2
Converges: |r| < 1.
Sum formula

9. Example 2B: Find the Sums of Infinite Geometric Series
Find the sum of the infinite geometric series, if it exists.
Evaluate.
Converges: |r| < 1.

10. Check It Out! Example 2a
Find the sum of the infinite geometric series, if it exists.
r = –0.2
Converges: |r| < 1.
Sum formula
125 6 =

11. Example 3: Writing Repeating Decimals as Fractions
You can use infinite series to write a repeating decimal as a fraction.
Example 3: Writing Repeating Decimals as Fractions
Write 0.63 as a fraction in simplest form.
Step 1 Write the repeating decimal as an infinite geometric series. =
Use the pattern for the series.

12. Example 3 Continued
Step 2 Find the common ratio.
|r | < 1; the series converges to a sum.

13.  Example 3 Continued Step 3 Find the sum. Apply the sum formula.
Check Use a calculator to divide the fraction

14. Recall that every repeating decimal, such as 0. 232323. , or 0
Recall that every repeating decimal, such as , or 0.23, is a rational number and can be written as a fraction.
Remember!