I am given a homework assignment on this question:
Show that if we add any number of incoming arcs, with any capacities, to the source node, the maximum flow value remains unchanged. Similarly, show that if we add any number of outgoing arcs, with any capacities, at the sink node, the maximum flow value remains unchanged.
While, it can be graphically proved with the help of an example, I am unsure of how to prove it systematically (like usual proofs in textbooks)