Is there a formal proof that demonstrates the complexity of solving the knapsack problem as NP-complete?

Yes, there is a formal proof that demonstrates the complexity of solving the knapsack problem as NP-complete. This proof involves reducing another known NP-complete problem, such as the subset sum problem, to the knapsack problem in polynomial time. This reduction shows that if a polynomial-time algorithm exists for solving the knapsack problem, then it can be used to solve all NP problems efficiently, implying that the knapsack problem is NP-complete.