Question

If the third term of an A.P. is 12 and the sixth term is 21. What is the common difference​

Answer 1

Answer:

ANSWER

Given that: T

3

=12 and T

6

=96

T

3

=12

⇒ar

2

=12 ......... (1)

T

6

=96

ar

5

=96 ........ (2)

divide (2) by (1):

ar

2

ar

5

=

12

96

∴r

3

=8

r

3

=2

3

∴r=2

Consider, ar

2

=12

a(2)

2

=12∴a×4=12∴a=3

Now, let us find S

9

.

S

n

=

r−1

a(r

n

−1)

S

9

=

2−1

3(2

9

−1)

=

1

3(512−1)

=3(511)

∴S

9

=1533

Answer 2

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