# Are these two derivatives the same with regards to TR?

Given that Marginal Revenue is change in Total Revenue:

TR = P*Q
P = f(Q)
TR = f(Q) * Q
MR = dTR/ dQ


Or

TR = P*Q
Q = f(P)
TR = f(P) * P
MR = dTR/dP


Which of these approach is right? My end goal is to relate revenue to elasticity. Does it matter how I arrive to it?

• It would be less ambiguous if you used different notations for different functions: $P=f(Q) \Leftrightarrow Q=f^{-1}(P) \Leftrightarrow Q=g(P)$ Feb 17 '20 at 10:03

Also, you can have multiple different elasticities. Elasticity for single variable function is just by definition $$EL=\frac{f’(x)x}{f(x)}$$. Depending on what types of elasticity you want to relate the total revenue and what exactly is your ultimate goal of your study either of the formulas could be used.