\(\frac{1}{2}\)+\(\frac{1}{4}\)+\(\frac{1}{8}\)+...+\(\frac{1}{2^n}\)=\(\frac{2^n-1}{2^n}\)

1.2.3+2.3.4+3.4.5+...+n(n+1)(n+2)=\(\frac{n\left(n+1\right)\left(n+2\right)\left(n+3\right)}{4}\)

1.2+ 2.3+ 3.4+ ... +n(n+1)=\(\frac{n\left(n+1\right)\left(n+2\right)}{3}\)

1²+3²+5²+ ... +(2n - 1)²= \(\frac{n\left(4n^2-1\right)}{3}\)

U_{1 = 2}

U_n+1 = \(\frac{1}{3}\)(U_n+1) Tìm U₄