Question

The sum of first 21 terms of the AP-5,13,21,
is equal to​

Answer 1

Answer:

GIVEN A.P. –

5 , 13 , 21 ,......

SUM OF FIRST 21 TERMS -

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

$s_{21} = \frac{21}{2} (10 + 20 \times 8)$

$s_{21} = \frac{21}{2} (170) = 1785$

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Answer 2

Hello !!

You can solve this question using the following method below, see the resolution below in the practice.

Find the common difference.

d = (An - Ak)/(n - k)

d = (13 - 5)/(2 - 1)

d = 8/1

d = 8

Find the sum of the first 21 terms of this arithmetic sequence.

Sn = (n/2) × [2A1 + (n - 1) × d]

S21 = (21/2) × [2(5) + (21 - 1) × 8]

S21 = (21/2) × [10 + 20 × 8]

S21 = (21/2) × [10 + 160]

S21 = (21/2) × 170

S21 = 3570/2

S21 = 1785

Therefore, with all this reasoning we have that the sum of the first 21 terms of this arithmetic sequence is 1785.

I hope I have collaborated !