Question 1088375
(1,2,3,...,38,39,40)

first term {{{a[1]=1}}}
second term {{{a[2]=3}}}
difference {{{d=2-1=1}}}

second term {{{a[2]=3}}}
third term {{{a[3]=3}}}
difference {{{d=3-2=1}}}


so, the terms have a common difference {{{d=1}}}, and this is indeed an arithmetic sequence. 

The last term in the partial sum will be:

{{{a[n]=a[1]+(n-1)d}}}...since {{{d=1}}}

{{{a[n]=a[1]+(n-1)}}}