Elasticsearch what are the formulas to calculate total shards

Question

I am new to Elasticsearch and I found out the standard way to calculate total shards is total shards=primary * (1+replicas)

So in this example

PUT /my_index/_settings
{
  "number_of_shards": 3,
  "number_of_replicas": 2
}

The answer gives as 9.

total shards=primary * (1+replicas)
9=3 * (1+2)

May I know that alternatively can I use below formula too since it gives the same answer as 9?

total shards= number_of_shards + (number_of_shards * number_of_replicas)

Answer 1

This is a math question and has nothing to do with ES.

primary * (1 + replica) = primary + (primary * replica)