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Question 12.1: The present value of a payment of £1 due in 8 years’ time is......

The present value of a payment of £1 due in 8 years’ time is given by the formula

PV  =  \frac{1}{(1  +  i)^8}

where i is the given interest rate. What is the rate of change of PV with respect to i?

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If we let

z = 1 + i              (1)

then we can write

PV =\frac{1}{z^8}   =  z^{-8}          (2)

Differentiating (1) and (2) gives

\frac{dz}{d i} = 1    \frac{dPV}{dz}  =  -8z^{-9}

Therefore, using the chain rule, the rate of change of PV with respect to i will be

\frac{dPV}{d i}  = \frac{dPV}{d z}\frac{dz}{d i}  =  -8z^{-9} = \frac{-8}{(1  +  i)^{9}}

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