Question

I just need someone to check my answer:

What is the sum of the arithmetic sequence 3, 9, 15..., if there are 24 terms?

a) 1452
b) 1728 (my answer)
c) 2028
d) 2268

Answer 1

1728

Explanation:

took the test

Answer 2

sum is n(an+a1)/2

an=a1+d(n-1)
common differnce is 6
first term si 3
an=3+6(n-1)
24th term
a24=3+6(24-1)
a24=3+6(23)
a24=3+138
a24=141

so nth term is 141
n=24
24(3+141)/2=12(144)=1728

answer is B