The function f$($x$,$ y$,$ r$)=(\{[1+(1$ x$)$y$]/(1+$ r$)\}1).$

Describes the net gain or loss of money invested, where x$=$ marginal tax rate, y $=$ annual effective yield on investment, and r $=$ annual inflation rate.

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Find the annual net gain or loss if money is invested at an effective yield of $7\%$ when the marginal tax rate is $25\%,$ and the inflation rate is $8\%,$ that is f$(0\mathrm{.}25,0\mathrm{.}07,0\mathrm{.}08).$

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Find the rate of change of gain $($or loss$)$ of money with respect to the marginal tax rate when the effective yield is $7\%,$ and the inflation rate is $8\%$

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Find the rate of change of gain $($or loss$)$ of money with respect to the effective yield when the marginal tax rate is $25\%$ and the inflation rate is $8\%$