write an A. P. in which a=10 and d is any natural number. find the sum of first 20 terms using formula.​

The Sum of first 20 terms of required AP is 2100.

Justification:

Let us suppose d is equal to 10,

then,

Structure of AP => a, a + d, a + 2d

AP => 10, 20, 30, 40...

Now,

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

Sn = 20/2(2 × 10 + (20 - 1)10)

Sn = 10(20 + 190)

Sn = 10 × 210

Sn = 2100.

Here, a = 10 . Let d= 5. The A.P. is 10,15,20..

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

Hence, the sum of first 20 terms is 1150.