function [v] = val_lagr(a,f,x)
n = length(a);
v = 0;
for k = 1:n
    l = 1;
    for i = 1:n
        if i ~= k
            l = l * (x-a(i))/(a(k)-a(i));
        end
    end
    v = v + l*f(k);
end
end