DERIV: Derivative function
syntax: DERIV(expr, varname, value) or
DERIV(expr, varname, value, prec)
returns the approximate derivative of <expr> with respect
to variable <varname> given the <value> at which to calculate
the derivative and the (optional) precision <prec> to calculate up to.
As <prec> gets smaller, the approximation usually gets more accurate
The central difference formula is used for this function