1. The problem statement, all variables and given/known data Show that every 2-tree(Definition 3.2) with n internal nodes has n+1 external nodes 2. Relevant equations A 2 tree is a tree which nodes have 2 children or no children. Internal nodes are nodes which have two children external nodes are leafs. 3. The attempt at a solution Proof base case n=1 Internal nodes=1 external nodes=2 Inductive step Inductive hypothesis: Let k internal nodes have k+1 external nodes so k+1 internal nodes will have (k+1)+1 external nodes.