One of the fundamental trigonometric functions. The derivative of sin, a.k.a. sine. Commonly abbrieviated cos. An operative function in many programming languages. Notations vary, i.e., cos-1x may mean 1/cosx or may mean arccosx. Useful in all branches of mathematics. Hyperbolic equivalent in cosh. Also equivalent to:
e^(i*x)+e^(-i*x)
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2
This means that cos(x)=cosh(ix).