The root will be the only node at level 1.

Each subsequent levels will be as full as possible i.e. 2 nodes at level 2, 4 nodes at level 3 and so on, i.e. in general there will be 21-1 nodes at level 1. Therefore the number of nodes from level 1 through level h-1 will be
1+2+22+23+ ... +2h-2=2h-1-1

So,the number of nodes in a balanced binary tree of height 'h' is 2h-1-1.
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