x = linspace(-1,1,1000);
plot(x, cos(50*acos(x)))
xi = cos([1 3 5 7 9]/10*pi)
yi = sin(pi*xi)
lp = lagrange(xi, yi)
plot(x, polyval(lp, x))
hold on; plot(x, sin(pi*x)); hold off;
plot(x, abs(polyval(lp, x) - sin(pi*x)))