Tính tổng Sn =1/1.4 + 1/4.7 + 1/7.10+...+1/(3n-2)(3+1), n thuộc N*
Giải thích
Ta có 13n−23n+1=1313n−2−13n+1
Suy ra Sn=131−14+14−17+...+13n−2−13n+1
⇔Sn=131−13n+1=n3n+1
Vậy Sn=n3n+1
Ta có 13n−23n+1=1313n−2−13n+1
Suy ra Sn=131−14+14−17+...+13n−2−13n+1
⇔Sn=131−13n+1=n3n+1
Vậy Sn=n3n+1