Question

find the sum of 20 terms of an ap:2,5,8,11...
step by step explanation ​

Answer 1

✬ S²⁰ = 610 ✬

Step-by-step explanation:

Given:

• AP series is 2 , 5 , 8 , 11

To Find:

• Sum of its 20 terms

Solution: Here ,

➮ a = 2 ( First term )

➮ d = 5 – 2 = 3 ( Common diffrence )

➮ n = 20 ( Number of terms )

As we know that

Sum of n terms of an AP a , a + d , a + 2d,......l is given by

★ Sn = n/2 { 2a + ( n – 1 ) d } ★

$\implies{\rm }$ S²⁰ = 20/2 {2 $\times$ 2 + (20 – 1)3

$\implies{\rm }$ S²⁰ = 10 { 4 + (19)3 }

$\implies{\rm }$ S²⁰ = 10 { 4 + 57 }

$\implies{\rm }$ S²⁰ = 10 $\times$ 61

$\implies{\rm }$ S²⁰ = 610

Hence, the sum of first 20 terms of the given AP is 610.

Answer 2

Answer:

Sum of 20terms of AP

SN = n/2( 2a +(n-1)d)

Here,,

A=2

D=3

N=20

S20=?

So,,,

$s20 = 20 \div 2 (2 \times 2 + (20 - 1) \times 3 \\ then \\ s20 = 10(4 + 19 \times 3) \\ 10(4 + 57) \\ 10 \times 61 \\ s20 = 610$

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