("*") - 4. Come Buy an Editorial [4 points] Use a combinatorial proof to show that for positive integers n and k: n+1 (n+1)(-1) k k Recall that in a combinatorial proof, we have three distinct parts (with an optional fourth): 1. Story: introduce a story that both sides of the equality count by different methods 2. LHS: prove that the left-hand side of the equality counts the story 3. RHS: prove that the right-hand side of the equality counts the story (It's a good practice to include a conclusion stating that you've proved the equality by combinatorial proof as both sides count the same story.)