Question 474057
23, 20, 17, 14, ... an arithmetic progression
{{{a1=23}}}
{{{d=a2-a1=20-23=-3}}}
Formula {{{Sn=(n/2)(a1+an)}}}, so {{{S21=(21/2)(a1+a21)}}}
{{{an=a1+(n-1)d}}}, then {{{a21=23+(21-1)*(-3)=23-60=-37}}}
{{{S21=(21/2)(23+(-37))=-147}}}