As suggested by Rich W., write them like any other testcases by getting expected results from Requirements.
Few conditions I would like to specify for writing test cases:
1. where input results in infinte results like division by zero.
2. rounding off (upto required decimal places) is correct when the result is recurring e.g 1/3. (If this is applicable)
3. when the calculation goes beyond the minimum & maximum range supported by your application.