Monopoly - Where is the Markup formula derived from? Problem: 

My lecturer wrote the marginal revenue formula before writing the markup formula, indicating some sort of similarity between the two. 

How it was written on the board:


  Market Power being $\text{P&gt;MC}$
  
  $\text{MR} = \text{P  (1+1/η) = MC}$ (This being profit maximising condition for a monopoly)
  
  Can be written as:
  
  $\text{MC/P = 1+1/η}$
  
  Markup: $\text{(P-MC/P)= -1/η}$


Question:

As much as I can see the similarity, I can't understand how he got to $\text{(P-MC/P)= -1/η}$ from $\text{MC/P = 1+1/η}$?

Index: 

P = Price,
MR = Marginal Revenue,
MC = Marginal Cost,
η = Elasticity
It is just a missing parenthesis issue. The expression

$$\text{(P-MC/P)= -1/η}$$  should have been written

$$\text{((P-MC)/P)= -1/η}$$