Question

find the sum of 1st 20terms of series 1+2+3+...​

Answer 1

Answer:

1+2+3...

It is sum of first n positive integers.

Sn = n(n+1)/2

= (20)(21)/2 = (10)(21) = 210

Answer 2

Answer:

n (n+1)/2

20(20+1)/2

10×21

210