Math help! Explain how to find the nth partial sum of a arithmeitc sequence

I try to figure out how to work things with problems with answers in the back of the book so heres an example:

8,20,32,44......., n = 10

the answer was:
620

I tried the sum of a finite arithmetic sequence formula:
S{n}=n/2(a{1} +a{2}]

ended up getting 160... or something

**Numbers in {x} here are under the number like exponents but at the bottom if you understand? xD

Thanks if you can help! :)

Good question!

To make learning math a bit easier, Dr. Pan (TucsonMathDoc) has recorded a YouTube video to help visually answer it.

Please comment on YouTube or Y!A and let her know if it helped.

Thanks!

S_n = n/2 [a1 + an ] or n/2[2a + (n-1)d ]

since you know a = 8, d = 20 - 8 = 12 and n = 10, use second formula

S_n = 10/2 [2*8 + 9*12]

= 5(16 + 108)

= 5*124

= 620