An interpolation function is unique. Also as far as i'm aware Lagrange interpolation gives the same polynomial as Newtons divided differences.
However when interpolating $\cos(x)$ at $x=\frac{-\pi}3;$ $0;$ $\frac\pi3$
Lagrange's gives $\frac{(-9x^2)}{(2\pi^2)}+1$
Newtons Gives $1-(\frac{3x}{2\pi})$
Are any of my initial assumptions wrong?
Could somebody explain this to me please?