HP HP-15C - Using an Approximate Slope

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Section 1: Using [SOLVE | Effectively
The relative field intensity is most negative at an angle of 28.0680C
from vertical.
Using an Approximate Slope
The derivative of a function can also be approximated numerically.
If you sample a function at two points relatively close to x (namely
x + A and x — A), you can use the slope of the secant as an
approximation to the slope at x:
f(x)
f(x + A)
f(x ~ A)
x-A
I
x + A