Assume the stock price follows geometric Brownian motion under the physical measure:
dS=μSdt+σSdW.
Let f(S,t) be the price of any derivative whose payoff depends only on S and t. Derive the Black-Scholes-Merton PDE
∂t∂f+rS∂S∂f+21σ2S2∂S2∂2f=rf
using a delta-hedging / no-arbitrage argument.