Let G be a connected planar graph with all circuits of length of at least k. Prove that the inequality e <= 3v - 6 with e = number of edge and v = number of vertex can be expanded into e <= (k/k-2)(v-2)
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Hint: look up the proof of the inequality $\displaystyle e \leqslant 3v - 6$. All it needs is a slight adaptation to the case where each face of the graph has at least k edges.
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