相同的数学表达式返回不同的结果

Question

I am baffled by this one.

So I have a console application doing a lot of calculations (trust me, thousands of them ). In one method, I have some parameters that need calculating, in different situations. For one of them, the mathematical expression is basically the same, only one difference in a term. Here is the code snippet along with all the lines between the 2 formulas in question, the Nq1 and Nq2 ones ( first formula of the code and last one to be more easy ):

//drained conditions
        Nq1 = Math.Round((Math.Pow(Math.E, Math.PI * Math.Tan(studiu.Fi * Constants.ConversionToDeg)) * Math.Pow((Math.Tan(45 + studiu.Fi / 2.00) * Constants.ConversionToDeg), 2)), 2);

        //Combination 2

        studiu.Fi = FiAfectat;


        //drained conditions
        Nq2 = Math.Round((Math.Pow(Math.E, Math.PI * Math.Tan(studiu.Fi * Constants.ConversionToDeg)) * Math.Pow((Math.Tan(45 + studiu.Fi / 2.00) * Constants.ConversionToDeg), 2)), 2);

The first formula returns 18.04 but the second one returns 0.01. How is this possible ? Only the studiu.Fi term differs, but not by that much ( 32 in the first case and 27 in the second ).

How can Nq1 be 18 and Nq2 be 0.01 ? Am I missing something here ?

Answer 1

Tan(x) is a periodic function that will change drastically for a small change in x. Since the two formulas only differ by a term inside the Tan function, this is likely your problem.

Also, it is likely that you should be using radians instead of degrees. If this is the case you should use a command that converts real numbers to radians instead of one that converts to degrees.