Page 417 - Excel for Scientists and Engineers: Numerical Methods

3 94 EXCEL: NUMERICAL METHODS

dydx
Returns the first derivative of a function y = F(x), represented by a formula in a cell, at
a specified value of x. Returns #DIV/O! error value if x = 0, in which case use the
optiona I a rg u ment scale-factor.

Syntax
dydx( expression, variable, scale-factor)
expression reference to a cell containing a formula (the function F(x)to be
differentiated)
variable cell reference corresponding to the independent variable x in the
function F(x)
scale-factor optional argument to be used when x is zero

Remarks
The argument expression can be either a reference to a cell that contains a formula,
or a name.
The argument reference can be either a reference to a cell, or a name.
Use the optional argument scale-factor to specify a suitable value of x to be used to
calculate Ax. For example, if the function requires values of x in the range -1 x lo5
to 1 x lo5, use 1E-5 for scale-factor.
The optional argument scale-factor can be either a number or a formula, or a
reference to a cell that contains a number or formula, or a name.
The function cannot handle implicit references; that is, a name or range reference
cannot be used for a range of values.
The workbook can be set to either RlC1- or Al-style.

Limitations
None of the precedent cells of the argument expression may contain references to
the argument reference.
The function cannot handle implicit references; that is, a name or range reference
cannot be used for a range of values.

Example
If cell C2 contains the formula =SIN(B2) and cell B2 contains the value 1, the formula
=dydx(C2,B2) returns the value 0.5403023062. The correct value is cos (1) =
0.5403023059 (5.8 x error).

See Also
d2ydx2