The proposition 6.9 states that: t is a proper binary tree with n nodes and h is the height.

The number of external nodes in T is at least h+1 and at most 2^h. or h>=h+1 and h<=2^h. I let e be the number of external nodes. I must prove this statement by induction. So i let P(h) be that statement. So obviously P(1) is true because that is 2 external nodes =h+1 and 2^h. I try solving for P(h+1) for the lower bound. so we assume that e>=(h+1)+1=h+1>=h+2 is true. so adding +1 to both sides of e>=h+1 i get e+1>=h+2. That can't be right.. I guess i'm confused about how to set this up.