format long

x=[0.6-cos(0.6) 0.7-cos(0.7) 0.8-cos(0.8) 0.9-cos(0.9)];
f_inverse_x=[0.6 0.7 0.8 0.9];

for k=1:4
  L(k)=1;
  for i=1:4
    if i!=k
      L(k)=L(k)*(0-x(i))/(x(k)-x(i));
    end
  end
end

s=0;
for j=1:4
  s=s+f_inverse_x(j)*L(j);
  end
  
  fprintf("x=%.15f\n",s);