Question

If a= 2 and 20th term is 62 ,then sum of first 20 terms of an AP is:​

Answer 1

Answer:

a=2

a20=62

a+19d=62

2+19d=62

19d=62-2

19d=60

d=60/19

Sn=n/2[2a+(n-1) d]

n=20

Sn=20/2[2.2+(20-1)60/19]

Sn=10[4+60]

Sn=640

Answer 2

☞Question.

✧If a= 2 and 20th term is 62 ,then sum of first 20 terms of an AP is?

☞Solution.

✧two formula used to get( sn )of any A.P

1. sn = n/ 2 [ a+ ( n-1 ) × d ]

2. sn = n/2 [ a + an( last term )]

☞Here we used formula no. ( 2 ) to find sum of A.P

→sn = $\sf\green{ n/2[ a + an ]}$

→sn = $\sf\green{ 20/2 [ 2 + 62 ]}$

→sn = $\sf\green{ 10 ( 64 )}$

→sn = $\sf\green{ 640}$

☞sum of first $\sf\green{20}$ of this A.P is

$\sf\green{ 640}$.