3.5 Exponential and Logarithmic Models – Compound Interest

  • If a principal P dollars is borrowed for a period of t years at a per annum (yearly) interest rate of r, expressed as a decimal, the interest I charged is (Simple Interest Formula)

We typically use the term payment period as follows:Annually = once per year

Semiannually = twice per year

Quarterly = four times per year

Monthly = 12 times per year

Daily = 365 times per year

  • The amount A after t years due to a principal P invested at an annual interest rate r compounded n times per year is .

Ex) A credit union pays interest of 8% per annum compounded quarterly on a certain savings plan. If $1000

is deposited in such a plan and the interest is left to accumulate, how much is in the account after 1 year?

We use the simple interest formula, . The principal P is $1000 and the rate of interest is 8% = 0.08. After the first quarter of a year, the time t is year, so the interest earned is

The new principal is . At the end of the second quarter, the interest on this new principal is

At the end of the third quarter, the interest on the new principal of is

Finally, after the fourth quarter, the interest is

After one year the account contains $1082.43

Using the compound interest formula we find we get the same answer (P=$1000, r=0.08, t=1, n=4)

  • Continuous Compounding

The amount Aafter tyears due to a principal Pinvested at an annual interest rate of rcompounded continuously is

Ex) On January 2, 2002, $2000 is placed in and IRA that will pay interest of 10% per annum compounded

continuously. (a) What will the IRA be worth on January 1, 2022?

(b) What is the effective rate of interest?

(a)The amount A after 20 years is

(b)First, we compute the interest earned on $2000 at r = 10% compounded continuously for 1 year

The interest earned is . Use the simple interest formula,

, with I = $210.34, P = $2000, and t = 1, and solve for r , for the effective rate of interest.

The effective rate of interest is 10.517%

  • Rate of interest required to double an investment

Ex) What annual rate of interest compounded annually should you seek if you want to double your

investmentin 5 years?

If P is the principal and we want P to double, the amount A will be 2P. We use the compound

interest formula with n = 1 and t = 5 to find r.

Substitute for known variables

Divide both sides by P

Take 5th root of both sides

The annual rate of interest needed to double the principal in 5 years is 14.87%

Ex) How long will it take for an investment to double in value if it earns 5% compounded continuously?

If P is the initial investment and we want P to double, the amount A will be 2P. We use the

continuous compounding formula with interest of r = 0.05.

continuous compounding formula

insert known variables

divide both sides by P

rewrite as a logarithm

ln(e) = 1 and power rule

solve for t

It will take about 14 years to double the investment.

(** Show how to solve using intersection of two graphs in calculator **)