Question

for what we should apply sn=n/2(2a+(n-1)d)​

Answer 1

Step-by-step explanation:

it is the formula to calculate sum of an ap

for example

the ap is 1,2,3,4,5,6

so for finding its sum we use this formula

Sn = 6/2 (2(1) + (5)(1))

=3(2 + 5)

=3(7)

=21

let's verify

1+2+3+5+6 = 21

so the above formula is verified