三角関数の円とグラフの関係です。
αの値を動かしてsinとcosの関係を探りましょう。

αは角度です。

α = 0 rad (slider from 0 to 6.283185307179586, step 0.017453292519943295)
f(x) = sin(x)
A = Intersect(f, xAxis, (0, 0))
B = Point(yAxis)
c = Circle(A, B)
D: (cos(α), sin(α))
g(x) = cos(x)
F = Point(c)
G = Intersect(f, xAxis, (0, 0))
e = Segment(D, G)
h = Segment(F, G)
α_1 = Angle(F, G, D)
i = Segment(G, D)
H: (0, sin(α))
C: (α, sin(α))
j = Segment(H, C)
k = Segment(D, H)
I: (cos(α), 0)
l = Segment(D, I)
E: (α, cos(α))