4.7 compound interest - poudre school...
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4.7 Compound Interest
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4.7 Compound Interest
Objective: Determine the future value of a lump sum of money.
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Simple Interest Formula:
I = PrtInterest
Principal
time in years
interest rate
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A credit union pays interest of 8% per annum compounded quarterly on a savings account. If $1,000 is deposited and the interest is left to accumulate, how much is in the account after 1 year?
I = Prt
I = ($1000)(0.08)( )14I = $20
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At the end of one quarter, we have earned $20 in interest.My principal is now $1020. So at the end of the second quarter, how much do I earn?
I = ($1020)(0.08)( )14
I = $20.40
Adding that interest to the money in my account, I now have $1040.40 6 months into the year.
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At the end of the third quarter, how much do I have?
I = ($1040.40)(0.08)( )14
I = $20.81
This brings my principal to $1,061.21.
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At the end of the fourth quarter, or at the end of the year what do I have?
I = ($1061.21)(0.08)( )14
I = $21.22
This brings the total in my bank account to $1,082.43 just by collecting interest.
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This process can be very lengthy especially if the interest were being compounded daily. This leads to a more general formula for calculating interest that is compounded.
A = P(1 + )ntr
n n = the number of compounds per year
Compounded interest formula:
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annually: once per yearsemiannually: twice per yearquarterly: four times per yearmonthly: 12 times per yeardaily: 365 times per year
Compounded interest is interest that is paid on principal and previously earned interest. Common forms of compounding are:
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Lets compare investments using different compounding periods.
Find the resulting amount of an investment of $3000 when it is compounded annually, semiannually, quarterly, monthly and daily at a rate of 4% for a period of 3 years.
A = (3000)(1 + )0.041
(1 3)
A = (3000)(1 + 0.04)3
A = (3000)(1.04)3
A = $3,374.59
A = P(1 + )ntr
n n = the number of compounds per year
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February 05, 2009
Feb 53:46 PM
Lets compare investments using different compounding periods.
Find the resulting amount of an investment of $3000 when it is compounded annually, semiannually, quarterly, monthly and daily at a rate of 4% for a period of 3 years.
A = (3000)(1 + )0.042
(2 3)
A = (3000)(1 + 0.02)6
A = (3000)(1.02)6
A = $3,378.49
A = P(1 + )ntr
n n = the number of compounds per year
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February 05, 2009
Feb 53:46 PM
Lets compare investments using different compounding periods.
Find the resulting amount of an investment of $3000 when it is compounded annually, semiannually, quarterly, monthly and daily at a rate of 4% for a period of 3 years.
A = (3000)(1 + )0.044
(4 3)
A = (3000)(1 + 0.01)12
A = (3000)(1.01)12
A = $3,380.48
A = P(1 + )ntr
n n = the number of compounds per year
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February 05, 2009
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Lets compare investments using different compounding periods.
Find the resulting amount of an investment of $3000 when it is compounded annually, semiannually, quarterly, monthly and daily at a rate of 4% for a period of 3 years.
A = (3000)(1 + )0.0412
(12 3)
A = (3000)(1 + 0.0033333333)
A = (3000)(1.0033333333)
A = $3,381.82
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A = P(1 + )ntr
n n = the number of compounds per year
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February 05, 2009
Feb 53:46 PM
Lets compare investments using different compounding periods.
Find the resulting amount of an investment of $3000 when it is compounded annually, semiannually, quarterly, monthly and daily at a rate of 4% for a period of 3 years.
A = (3000)(1 + )0.04365
(365 3)
A = (3000)(1 + 0.0001095890411)1095
A = (3000)(1.0001095890411)
A = $3,382.47
1095
A = P(1 + )ntr
n n = the number of compounds per year
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Compounding Interest Continuously:A = Pert
A = (3000)e (0.04 3)A = Pert
A = (3000)e 0.12
A = $3,382.49
What if the investment is compounded more often than daily?
Find the resulting amount of an investment of $3000 when it is compounded continuously at a rate of 4% for a period of 3 years.
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How much principle is needed now to have $4000 after 2 years at 6% compounded quarterly?
4000 = P(1 + )0.064
(4 2)
4000 = P(1 + 0.015)8
4000 = P(1.015)8
P = $3,550.84
A = P(1 + )ntr
n n = the number of compounds per year
4000 = P(1.126492587)
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How long will it take for an investment $250 to reach $675 in value if it earns 9% compounded monthly?
A = P(1 + )ntr
n n = the number of compounds per year
675 = (250)(1 + )0.0912
(12 t)
675 = (250)(1 + 0.0075)
675 = (250)(1.0075)
2.7 = (1.0075)
12t
12t
log 1.00752.7 = 12t
= 12tlog 2.7
log 1.0075
132.9295772 = 12t
t ≈ 11.08 years
12t
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How long will it take for an investment to double in value if it earns 5% compounded continuously?
A = Pert
2P = Pe 0.05t
2 = e0.05t
ln 2 = ln e0.05t
ln 2 = 0.05t ln e
ln 2 = 0.05t
t = 13.86 years
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page 322(4, 5, 8, 9, 12, 14, 16,
18, 19, 28, 32, 36, 37, 48)
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