Câu trả lời:
\(S=\frac{3}{2^2}.\frac{8}{3^2}.\frac{15}{4^2}.........\frac{399}{20^2}=\frac{1.3.2.4.3.5............19.21}{\left(2.3.4.....20\right)\left(2.3.4........20\right)}=\frac{\left(1.2.3.......19\right)\left(3.4.5......21\right)}{\left(2.3.4....20\right)\left(2.3.4......20\right)}=\frac{1.21}{20.2}=\frac{21}{40}\)