Question 1181059


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

Find the sum of the first four terms of the arithmetic sequence: {{{7}}}, {{{15}}}, {{{23}}}, . . . .

first term

{{{a[1] =7}}}

common difference

{{{d=8}}}

fourth term

{{{a[4]=a[1]+(4-1)d}}}
{{{a[4]=7+3*8}}}
{{{a[4]=31}}}

{{{S[4] = (4/2) ( 7+ 31) }}}
{{{S[4] = 76}}}


answer: d. {{{76}}}