function yint = Lagrange(x,y,xx)
x=[0 30000 60000 90000 120000];
y=[9.810 9.7487	9.6879 9.6278 9.5682];
xx=55000;
n = length(x);
if length(y) ~=n, error ('x and y must be same length'); end
s = 0;
for i = 1:n
product = y(i);
for j = 1:n
if i ~= j
product = product * (xx-x(j))/(x(i)-x(j));
end
end
s = s + product;
end
yint = s;