Question 709273
The sequence of terms uses difference of 2.  The beginning term is 3.


The general term of the sequence, {{{a[n]=3+(n-1)*2}}}


The summation for n terms of an arithmetic sequence is
{{{S=(n/2)(a[1]+a[n])}}}, using as indicated, the first and last terms in the range.


{{{(8/2)(3+17)=80}}}