Question 936142

find the sum of the first 20 terms in the series 1,5,9,13,17...
<pre>
To determine the sum, use the formula for the sum of an AP, with number of terms, 1st term, and common difference (d), known.
Formula: {{{S[n] = (n/2)(2a[1] + (n - 1)d)}}}
{{{S[n] = (20/2)(2(1) + (20 - 1)4)}}} ------ Substituting 20 for n, 1 for {{{a[1]}}}, and 4 for common difference , or d
{{{S[n] = 10(2 + (19)4)}}}
{{{S[n] = 10(2 + 76)}}}
{{{S[n] = 10(78)}}}
Sum of 1st 20 terms, or {{{highlight_green(S[n] = 780)}}}