Question

find the sum of the following AP . 2,4,8,6...20 terms​

Answer 1

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given :

• a = 2
• d = 2

➪ An = 20

➪ 30 = 2 + 2n - 2

➪ 30 = 2n

➪ n = 30/2

➪ n = 15

_____________________

now,

Sn = n/2(a + l)

➪ Sn = 15/2 ( 2 + 20)

➪ Sn = 15/2 × 22

➪ Sn = 15 × 11

➪ Sn = 165

so, sum of the terms of AP is 165

Answer 2

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a = 2

d = 2

n = 20

$S_{n} = \frac{n}{2}[2a + (n - 1)d]$

$= \frac{20}{2}[2(2) + (20 - 1)2]$

$= 10[4 + (19)2]$

$= 10 [4 + 38 ] \\ = 10[42] \\ = 420$