Computing areas under the Normal Curve with the TI-81

The program listed below will allow the user to find areas under the normal curve for intervals ranging from -4.5 to 4.5.  The results are reported as percentages (0 to 100%) rounded to two places past the decimal (e.g. 68.29%).
The results are fairly accurate, with worst-case errors of 0.02%.

For the mathematically inclined, the algorithm simply evaluates an eighth-degree polynomial approximation of the cumulative density function at the two endpoints and subtracts.  The polynomial approximation is based on mutually orthogonal polynomials on [-4.5,4.5].

Just enter this program in your TI-81 and run.  Please proofread your typing carefully; being off by just one character will likely cause serious errors.

Last Modified May 5, 1999.
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