why does cosine theta equals 3 not a solution?

Question

I know that cosine theta equals 3 is not a solution but I don't understand why

Answer 1

The cosine of an angle cannot be outside of the range from -1 to 1.  Recall the definition of the cosine of an angle.  It is the ratio of the adjacent side to the hypotenuse.  This ratio can never exceed 1, and can never be below -1.