Question

Identify the indicated sum for the geometric series.
S, for 3 - 6 + 12 – 24 + .....

Answer 1

Answer:

Here,

3

,

(

−

6

)

,

12

,

(

−

24

)

,

...

is the geometric sequn.

w

h

e

r

e

,

1

s

t

t

e

r

m

=

a

1

=

3

and

common ratio

r

=

−

6

3

=

12

−

6

=

−

24

12

=

−

2

and

S

9

=

?

⇒

n

=

9

We know that,

The sum of first n term of Geometric series is :

S

n

=

a

1

(

r

n

−

1

)

r

−

1

Putting ,

a

1

=

3

,

r

=

−

2

and

n

=

9

S

9

=

3

[

(

−

2

)

9

−

1

]

(

−

2

)

−

1

∴

S

9

=

3

[

−

512

−

1

]

−

3

⇒

S

9

=

3

(

−

513

)

−

3

⇒

S

9

=

513

Step-by-step explanation:

Answer 2

Answer:

Step-by-step explanation:

c.r. r=-6/3=-2

$S_{n}=3\frac{(-2)^n-1}{-2-1} =-(-2)^n+1\\let~n\rightarrow \infty\\then S_{\infty} \rightarrow \infty~or~ -\infty$