Prove that, if all perfect matchings in G are pairwise disjoint, than every two perfect matchings contain the edge set of a hamiltonian cycle in G.

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I have no idea how to prove this one. It looks like common sense, but I don't know hot to proceed with the proof.

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Think it through in two steps:

  1. What does the union of two disjoint perfect matchings $M_1 \cup M_2$ look like in general?
  2. If that union is not a Hamiltonian cycle, can you find inside it a third perfect matching $M_3$ different from both $M_1$ and $M_2$? (Yes, you can; how?)

Because if you can, then $M_3$ is certainly not disjoint from either $M_1$ or $M_2$, and so not all matchings are pairwise disjoint.

Conversely, if all matchings are pairwise disjoint, then we can never find this $M_3$, which means that $M_1 \cup M_2$ must be a Hamiltonian cycle.