The calculus of the trigonometric functions
The author points out that at the beginning of calculus there was quite a different treatment of the basic transcendental functions. Exponential and logarithmic functions have been presented from the origin while the trigonometric functions appear only 70 years later in the work of Euler in 1739. A reason for this delay could be that sine and cosine only were considered as geometric objects and that no applications of such functions have been seen. Euler recognized them as solutions of differential equations and he did ``present the first principles and reveal the highest uses in various parts of mathematics through the most impressive examples'' (cited from an abstract of a paper of Euler, published in 1754).
scientific article; zbMATH DE number 4059335