Question 1037724
<pre>
The sum of the first n terms

{{{S[n]=expr(n/2)(2a[1]+(n-1)d)}}}

Where a<sub>1</sub> = first term = 7
and d = common difference = a<sub>2</sub>-a<sub>1</sub> = 10-7 = 3 = a<sub>3</sub>-a<sub>2</sub> = 
13-10 = 3 = a<sub>4</sub>-a<sub>3</sub> = 16-13 = 3

{{{S[n]=expr(n/2)(2a[1]+(n-1)d)}}}

{{{(matrix(10,1,

The,sum,of,the,terms,from,a[13],to, a[21],inclusive))}}} {{{""=""}}} {{{(matrix(10,1,

The,sum,of,the,terms,from,a[1],to, a[21],inclusive))}}}{{{""-""}}}{{{(matrix(10,1,

The,sum,of,the,terms,from,a[1],to, a[12],inclusive))}}}{{{""=""}}}

{{{S[21]}}}{{{""-""}}}{{{S[12])}}}{{{""=""}}}

{{{expr(21/2)(2(7)+(21-1)3)}}}{{{""-""}}}{{{expr(12/2)(2(7)+(12-1)3)}}} {{{""=""}}}

{{{expr(21/2)(14+(20)3)}}}{{{""-""}}}{{{6(14+(11)3)}}} {{{""=""}}}

{{{expr(21/2)(14+60)}}}{{{""-""}}}{{{6(14+33)}}} {{{""=""}}}

{{{expr(21/2)(74)}}}{{{""-""}}}{{{6(47)}}} {{{""=""}}}

{{{777}}}{{{""-""}}}{{{282}}} {{{""=""}}}

{{{495}}} 

Edwin</pre>