Los elementos de mayor grado pueden obtenerse mediante polinomios de Lagrange: $$ \begin{equation*} \ell = \prod_{i=0, i \neq j}^{k} \frac{\xi - \xi_{i}}{\xi_{j} - \xi_{i}} \end{equation*} $$
Usando la fórmula $$ \begin{align*} N_{1} &= \frac{\xi - 1}{-1 - 1} = \frac{1}{2} - \frac{1}{2} \xi \\ N_{2} &= \frac{\xi - (-1)}{1 - (-1)} = \frac{1}{2} + \frac{1}{2} \xi \end{align*} $$
Usando la fórmula $$ \begin{align*} N_{1} &= \frac{\xi - 0}{-1 - 0} \frac{\xi - 1}{-1 - 1} = -\frac{1}{2} \xi + \frac{1}{2} \xi^{2} \\ N_{2} &= \frac{\xi - (-1)}{0 - (-1)} \frac{\xi - 1}{0 - 1} = 1 - \xi^{2} \\ N_{3} &= \frac{\xi - 0}{1 - 0} \frac{\xi - 1}{1 - 1} = \frac{1}{2} \xi + \frac{1}{2} \xi^{2} \end{align*} $$