Point B is contructed as a point that has an x-coordinate equal to the x-coordinate of point A and a y-coordinate equalt to the slope of the line tangent to f(x) at that x value (in other words, the derivatve of f(x) at that point).

Drag point A and watch the function that gets traced out by point B as you change the x-value of f(x). What function does the point trace? What function is the derivative of f(x)=sin(x)?