Question

What is the sum of the first 27 terms of the sequence 2.5+5.5+8.5+11.5?

Answer 1

It's an arithmetic sequence with:

`a_1=2.5\\\\d=5.5-2.5=3`
The formula of the sum of the n terms of an arithmetic sequence:

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

`n=27;\ a_1=2.5;\ d=3\\\\S_(27)=(2\cdot2.5+(27-1)\cdot3)/(2)\cdot27=(5+26\cdot3)/(2)\cdot27=1,120.5`