For 10 points, name this function whose reciprocal is secant, defined as adjacent over hypotenuse.
In information retrieval, this function's namesake similarity is a common method of quantifying how well documents are related. The Dottie number is this function's unique real fixed point. This function is used in the real parts of both De Moivre's formula and Euler's formula. When graphed, an analogue of this function forms a catenary curve. The dot product of two vectors multiplies those vectors' norms by this function of the (*)  angle between them. When  a ,  b , and  c  are the sides of a triangle, the fact that " c -squared equals  a -squared plus  b -squared minus two- ab  times  this function  of the angle opposite  c " is known as the law of this function. For 10 points, name this function whose reciprocal is secant, defined as adjacent over hypotenuse.
In information retrieval, this function's namesake similarity is a common method of quantifying how well documents are related.
cosine  [or  cos ; accept  hyperbolic cosine  or  cosh ; accept answers like  cosine  of  x ]