Here we have presented 3, 4, and 5 points explicit finite difference
formulae along with the error terms, for approximating the first and higher order
derivatives for unequally spaced data. By differentiating the equation (1), finite
difference formulae for higher order derivatives can also be obtained. These
formulae can be used directly to solve the ordinary and partial differential
equations and will serve to approximate the derivatives at the unequally spaced
grid points. A scientific program of the above formulae will facilitate the use of
any finite difference formulae of desired derivatives.