Firstly, an adjacency is a bi-directional concept. So if you have an adjacency between A and B, it's better to count it as one adjacency rather than two adjacencies (one for A and one for B).
Therefore, in a network of 5 routers with no DR/BDR, the number of adjacencies is: n(n-1)/2 = 10.
In a network of 5 routers with a DR/BDR, the number of adjacencies is: 2(n-2)+1 since each router has an adjacency with the DR and the BDR. In addition, the DR is adjacent to the BDR. This gives you 7 adjacencies for 5 router network.
So your calcuations are pretty much correct - except that you have counted each adjancency twice.
Hope that helps - pls do rate the post if it does.
It's not really about perspective. In an exam situation I would go for the answer I have given. Consider the case where you have only two routers with OSPF running between them. According to your perspective, you would have two adjacencies between them, which is not really correct. If you told someone that you had two adjacencies between two routers, they would immediately assume that both router had two links between them, each running OSPF. So the correct answer is that there is one adjacency in this case.
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