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