Past exam of the mathematics course of the University of Cambridge / 2002 / ib / Paper 2 / 14B / b / Solution

For $n\ge 1$, let $p_L,p_R$ be the interpolation polynomials of degree at most $n-1$ on nodes $x_0,\dots ,x_{n-1}$ and $x_1,\dots ,x_n$, respectively. The polynomialhas degree at most $n$. At $x_0$ it equals $p_L(x_0)$; at $x_n$ it equals $p_R(x_n)$. At each common node both interpolants equal $f$, and the displayed weights sum to one, so $p$ also equals $f$ there. Thus it is the full interpolant by uniqueness.

Its coefficient of $x^n$ is the difference of the leading coefficients of $p_R,p_L$ divided by $x_n-x_0$. By the definition of a divided difference,