According to "Help":
If the function contains more than one variable, type a comma after series, and then type a comma-separated list of variables around which you want to expand. By default, PTC Mathcad expands the function about the point 0 and returns the first six terms of the series. You can change the default behavior by including arguments after series. To expand around a point other than 0, specify a value for the variable or variables after the keyword series, using the Boolean equal operator.
