Question

Tenth term of an AP is 21 and the sum of its first 10 terms is 120. Find the sum of first 20 term of AP.
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Answer 1

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If the 10th term of an A.P is 21 and the sum of its first ten terms is 120, find its nth term.

November 22, 2019avatar

Deepshi Thinakaran

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ANSWER

Given,

a

10

=21

a+(10−1)d=21

a+9d=21....(1)

S

10

=120

2

10

[2a+(10−1)]d=120

2a+9d=24......(2)

solving (1) and (2), we get,

a=3

from (2), we have,

d=2

a

n

=a+(n−1)d

a

n

=3+(n−1)2

∴a

n

=2n+1

Answer 2

Step-by-step explanation:

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