C# Math.Acos How prevent NaN

Question

I'm trying to make a code to answers a Math Function that includes Acos "Cos^-1(A)", where A is a double -10.0000 <= A <= 10.0000, I had try several numbers in that range, and most of those give me a NaN as return that make all the operation Fails.

Edited: As the documentation said, with Math.Acos is I put a number out of the range -1<=d<=1 will return a NaN, I were attempting to make a function that make the same, step by step to allow those values.

Answer 1

As MSDN says, the argument to the Math.Acos function must be in range [-1,1]. Otherwise, the result will be NaN.

Answer 2

Your expectation that it should return something for those values is incorrect.

The arccosine function answers the question "what angle has this cosine?". But there is no value that you can give to the cosine function to yield a result less than -1 or greater than 1. We say that its "range is [-1, 1]".

Since there's no angle with a cosine value outside this range, it is not possible to give you an angle back when you ask Acos for one. So instead it gives you the answer of NaN.

The documentation for Math.Acos says this too:

Parameters:

d: (System.Double) a number representing a cosine, where d must be greater than or equal to -1, but less than or equal to 1.