5.53 How to generate sequence sum symbolically

n := 7; 
f:=sum('a[k]*b[k]','k'=1..n);
 

\[ a_{{1}}b_{{1}}+a_{{2}}b_{{2}}+a_{{3}}b_{{3}}+a_{{4}}b_{{4}}+a_{{5}}b_{ {5}}+a_{{6}}b_{{6}}+a_{{7}}b_{{7}} \]