To Define a Curve's Formula
 Open the Curve by Formula dialog ( split button).
 In the Curve field, key in the curve's name.
 Define the equations, finishing each with a semicolon ;.
 (Optional) Place a curve to test the equation.

Save the equation.
Note: Some mathematical knowledge is needed to define a new curve. A curve is defined by the parametric equations for the x, y, and z coordinates of the curve. These formulas give the value of the articular coordinate as a parameter t that is between zero and one (0.0 < t < 1.0). This is the standard parametric form of a curve described in mathematical text books.