Question 411231


To find the sum of a certain number of terms of an arithmetic sequence:

{{{Sn=n(a1+an)/2}}}

where {{{Sn}}} is the sum of {{{n}}} terms (nth partial sum),  {{{a1}}} is the first term,  an is the nth term


given: {{{a1=2}}} and {{{a40=80}}}  

to find: the sum of a2+a4+a6+a8+...a40

{{{Sn=n(a1+an)/2}}}


{{{S(40)=40(2+80)/2}}}



{{{S(40)=40*82/2}}}


{{{S(40)=40*41}}}



{{{S(40)=1640}}}