What is the hyp button on the calculator for?

[세이지]

It's for calculating hyperbolic trig functions.

Press HYP before pressing SIN to calculate the hyperbolic sine, Press HYP before COS or TAN to calculate the hyperbolic cosine or the hyperbolic tangent.

Hyperbolic functions once were just a mathematical curiosity; but then they turned up in the solutions to certain advanced scientific and engineering problems.

It's kind of silly that calculators have those buttons anymore. Scientists and engineers don't use scientific calculators for that kind of work: They use computer-based mathematics software packages that are vastly more powerful and functional than any hand-held device.