using the formula for the sum of an arithmetic series, what is the sum of the first 20 terms?

Answer 1

1+2+3 + ... + n
= n(n+1) / 2

For 20 terms this is 20*21/2 = 210

Answer 2

let d be the arithmetical ratio, u1 is the first number, n is number of terms ( here is 20 )
we have these formulas
S = (u1 + u_n)*n/2
u_n = (n-1) * d + u1
so S = [2u1 + (n-1)d]*n/2

Answer 3

Sn=n/2[2a+(n-1)d]
put n=20
S20=20/2[2a+(20-1)d]
= 20a+190d