f(x) = cos(x)
a = 2.387814692820413 (slider from -6.283185307179586 to 6.283185307179586, step 0.0010)
A: (a, f(a))
B: (a + 0.01, f(a + 0.01))
b = Line(A, B)
C: (a, Slope[b])