Past exam of the mathematics course of the University of Cambridge / 2021 / ii / Paper 3 / 29K / b / Solution

If ${\theta }_n$ shares are held during period $n$, the remaining wealth $X_{n-1}-{\theta }_n{S}_{n-1}$ is in the bank account. Self-financing therefore giveswhich rearranges to the required equation.

Solved by gpt-5.6-sol high.