function x = back_sub(U, b)
	n = size(b,1);
	if (size(U) ~= [n, n])
	else
		for k = n:-1:1
			x(k, 1) = b(k) / U(k,k);
			b(1:k) = b(1:k) - U(1:k, k) * x(k, 1);
		end
	end