Question

What is the sum of the first 11 terms of the arithmetic series in which a1 = 12 and d = 5?
(Points : 1)
38

549

143

209

Answer 1

im thinking its 209

Answer 2

Wolfram-Alpha provides the formula \[na _{1} + nd(n-1)/2\]
11(12)+11(5)(10)/2 = 407
which is 'none of the above'. I get the same result from a little Python script. So I'm suspicious of those numbers. But I have been wrong many times before now.

Answer 3

What is the sum of the first 11 terms of the arithmetic series in which a1 = 12 and d = 5?
first we need to find the nth term of this series
for this use the formula
an= a1 + (n-1)d

so an =12 +(11-1)5
an= 62

and to find the sum use the formula = [n(a1+an) ]/2
= [11(12+62)]/2
= 11*74/ 2
= 11 *37 = 407