C_f

f(x) = If[0 ≤ x ≤ 1, 1 - x^(2)]
a = 0.54 (slider from 0 to 1, step 0.01)
O = (0, 0)
M: (a, f(a))
g = Tangent(M, f)
Q = Intersect(g, xAxis)
P = Intersect(g, yAxis)
t1, p, o, q = Polygon(O, Q, P)