Solution

ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2013/p/1/c/solution

Integrate the Lagrange interpolation polynomial and exchange the integral with the finite sum:
Therefore the interpolatory quadrature weights are
The resulting quadrature rule is exact for every polynomial of degree at most , since its interpolant is itself. For a general continuous , replacing by its interpolant supplies the approximation.

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