66. The Rule of 70 A well-known rule in the world of finance is that an annual interest rate of R% will double an investment in approximately 70/R years. Assume that the money is com- pounded continuously so that you can use the Pen formula. (a) Solve the equation Pe" = 2P to find t as a function of r. (b) If r = R%, write t as a function of R. (c) If the money is not compounded continuously, the dou- bling time will be a little longer than the answer in (b). Approximate the typical doubling time by increasing the numerator by 1. This should explain the Rule of 70!

Calculus: Early Transcendentals
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ISBN:9781285741550
Author:James Stewart
Publisher:James Stewart
Chapter1: Functions And Models
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Problem 1RCC: (a) What is a function? What are its domain and range? (b) What is the graph of a function? (c) How...
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66. The Rule of 70 A well-known rule in the world of finance
is that an annual interest rate of R% will double an investment
in approximately 70/R years. Assume that the money is com-
pounded continuously so that you can use the Pe" formula.
(a) Solve the equation Pet = 2P to find t as a function of r.
(b) If r = R%, write t as a function of R.
(c) If the money is not compounded continuously, the dou-
bling time will be a little longer than the answer in (b).
Approximate the typical doubling time by increasing the
numerator by 1. This should explain the Rule of 70!
Transcribed Image Text:66. The Rule of 70 A well-known rule in the world of finance is that an annual interest rate of R% will double an investment in approximately 70/R years. Assume that the money is com- pounded continuously so that you can use the Pe" formula. (a) Solve the equation Pet = 2P to find t as a function of r. (b) If r = R%, write t as a function of R. (c) If the money is not compounded continuously, the dou- bling time will be a little longer than the answer in (b). Approximate the typical doubling time by increasing the numerator by 1. This should explain the Rule of 70!
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