Ta có công thức: \(1^2+3^2+5^2+...+\left(2n-1\right)^2=\frac{n\left(2n-1\right)\left(2n+1\right)}{3}\)
\(lim\frac{n\left(2n-1\right)\left(2n+1\right)}{3n^3}=lim\frac{1\left(2-\frac{1}{n}\right)\left(2+\frac{1}{n}\right)}{3}=\frac{1.2.2}{3}=\frac{4}{3}\)