Prove that a planar bipartite graph on n nodes has at most 2n−4 edges.

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I know that we have to use Euler's formula ( v−e+f=2) but I don't understand how f = e/2.

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Can you show that a planar graph has at most $3V-6$ edges? You will use the fact that the bounded faces of such a graph are (at least) triangles.

Hint: A bipartite graph has no odd cycles. This means that each bounded face instead touches at least $4$ edges.