คำศัพท์คณิตศาสตร์
It is clear that the sin and cos of a real number is a real number, but what about the sin and cos of a number that is pure imaginary? From Eqs. (1.10) and (1.11) it follows that the sin and cos of a pure imaginary number is ... drumroll, please ... real! This was the inspiration for defining hyberbolic cos and sin. They are defined by simply erasing the “i’s” in Eqs. (1.10) and (1.11):