Question

if the nth term of an AP is (2a+1) find the sum of first 20 terms.

Answer 1

EXPLANATION

• GIVEN

Nth term of an Ap = ( 2a + 1 ).

TO Find SUM OF FIRST 20 TERM.

Nth term = (2a + 1 )

=> let a = 1

2(1) + 1

2 + 1 = 3

=> let a = 2

2(2) + 1

4 + 1 = 5

=> let a = 3

2(3) + 1

6 + 1 = 7

=> let a = 4

2(4) + 1

8 + 1 = 9

series is written as = 3,5,7,9,.....

first term = a = 3

common difference = d = b - a

5 - 3 = 2

sum of Nth term of Ap

$sn \: = \frac{n}{2}(2a \: + (n \: - 1)d)$

$S20 = \frac{20}{2}(2(3) + (20 - 1)2 )$

$10(6 + 38)$

$10(44)$

$S20 = 440$

Hence,

sum of first 20 term = 440

Answer 2

The above mentioned answer is correct.

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