Question

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

1) 2,028
2) 1,452
3) 1,1728
4) 2,268

Answer 1

1,728

Explanation:

Answer 2

`a_1=3;\ a_2=9;\ a_3=15;\ ...\\\\d=a_2-a_1\to d=9-3=6`

The formula of the sum of the arithmetic sequence:


`S_n=(2a_1+(n-1)d)/(2)cdot n`

Substitute:


`a_1=3;\ d=6;\ n=24\\\\S_(24)=(2\cdot3+(24-1)\cdot6)/(2)\cdot24=(6+23\cdot6)\cdot12=(6+138)\cdot12=144\cdot12=1,728`

Answer: 1,728.