Given $f(x)=\sqrt[3] x$, find an approximation of $\sqrt[3]{20}$ using Lagrange interpolation method.
$x_0=0$, $x_1=1$, $x_2=8$, $x_3=27$ and $x_4=64$
$f(x_0)=0$, $f(x_1)=1$, $f(x_2)=2$, $f(x_3)=3$ and $f(x_4)=4$
What I've done is, calculating $p(20)=-1.3139$ which is obviously wrong. Why is that?