Calculus of Trigonometry
From conversations with Sue VanHattum about deriving the derivative of sine or cosine with infinitesimals. See:
http://thephysicsvirtuosi.com/posts/trigonometric-derivatives.html
Adjust point B to change d(alpha), the change in angle alpha. Compare the change in sine and cosine to the change in angle. As B gets closer, do the approximations get better?