 Solution

For a fixed $t\in [0,1]$, De Casteljau's algorithm first takes the same fractional position along both adjacent sides of the control polygon:It then applies another linear interpolation with that same $t$:This is the degree-two Bernstein basis representation of a quadratic Bézier curve. Its coordinate polynomials areThe degree is exactly two, since its second derivative $(-4,-4)$ is nonzero. As a check on the construction, at $t=1/2$ the two intermediate points are $(-1,1/2)$ and $(0,1/2)$ and their midpoint is $F(1/2)=(-1/2,1/2)$.