See Fig.
y = x2 is parabola (C1) with vertex (0, 0) and passing through B(1, 1), y = (1 - x)2 is parabola (C2) with vertex at A(1, 0) and passing through D (0, 1).
y = 2x(1 - x) = 2x - 2x2 = - 2(x - 1/2)2 + 1/2 is parabola with vertex at H(1/2, 1/2).