Casio fx-9860GII - 1.21 Summation of a series

Using a graphic display calculator
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Casio fx-9860GII
Sequences and series
1.21 Summation of a series
Example 24
Find the sum of the fi rst 20 terms of the arithmetic sequence 4, 7, 10, 13, …
The kth term of an arithmetic sequence is u
k
= u
1
+ (k − 1)d
In this example u
1
= 4, d = 3 and n = 20.

å
n
k
Sk
20
1
4( 1)3
Press
MENU
. You will see the dialog box as shown on the right.
Choose 1: RUN·MAT and press
EXE
.
Press
F4
MATH |
F6
|
F2
Σ(
The template matches the written Sigma formula.
Enter the variables, values and the function as they are written.
Use the
keys to move around the template.
The sum of the terms of the sequence is 650.
Example 25
Find the sum of the fi rst 12 terms of the geometric sequence
31
1
3
1
9
, , , , ...−−
The kth term of a geometric sequence is u
k
= u
1
⋅ r
k −1
In the example u
1
= 3,
r =−
1
3
and n = 12.
1
12
1
1
3
3
k
n
k
S


å
æö
÷
ç
÷
ç
÷
ç
èø
Press
MENU
. You will see the dialog box as shown on the right.
Choose 1: RUN·MAT and press
EXE
.
Press
F4
MATH |
F6
|
F2
Σ(
The template matches the written Sigma formula.
Enter the variables, values and the function as they are written.
Use the
keys to move around the template.
The sum of the terms of sequence is 2.25, to 3 signifi cant fi gures.
25