The formula to find any term of an an arithmetic sequence is given by
[tex]a _{n} [/tex]=[tex]a _{1} [/tex]=(n-1)d,
where we have first term [tex] a_{1} [/tex]=8,
and d which is the common difference , and n is the term we need to find (in our case is 20)
the difference d=3, as you see each number is the previous minus 3
for S20=8+(20-1)*(-3)=8+(19)*(-3)=8-57=-49
S20=-49, this the 20th term
