Example

Example 10.43: Solving for the Rate of Continuous Compound Interest

To find the rate of growth required for a continuously compounded investment to reach a specific future value, apply the formula A=PertA = Pe^{rt}. For example, if an initial investment of 10,00010{,}000 is made on a child's first birthday and is intended to grow to 50,00050{,}000 by their 1818th birthday (a period of 1717 years), substitute A=50,000A = 50{,}000, P=10,000P = 10{,}000, and t=17t = 17 into the formula: 50,000=10,000er1750{,}000 = 10{,}000e^{r \cdot 17}. Divide both sides by 10,00010{,}000 to isolate the exponential expression, yielding 5=e17r5 = e^{17r}. Next, take the natural logarithm of both sides to get ln5=lne17r\ln 5 = \ln e^{17r}. Using the Power Property and the fact that lne=1\ln e = 1, this simplifies to ln5=17r\ln 5 = 17r. Finally, divide by 1717 to solve for rr: r=ln5170.095r = \frac{\ln 5}{17} \approx 0.095. The investment requires an approximate growth rate of 9.5%9.5\%.

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Updated 2026-05-25

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