X4/4!
This function's Taylor series approximation begins 1 - X2/2! + X4/4! ("One minus X squared over two factorial plus X to the fourth over four factorial"). In Euler's Formula, this function added to i(sin(x)) ("I times the sine of X") is equal to eix ("E to the I X"). This function is also equal to the square root of 1-sin2 (theta) ("One minus sine squared of theta") and approaches zero as theta approaches p/2 ("pi over two"). Name this function equal to a triangle's adjacent side over its hypotenuse.
This function's Taylor series approximation begins 1 - X2/2! +
cosine