[Maths Class Notes] on Compound Interest Formula Pdf for Exam

Compound Interest is an interest earned on the original principal and the interest accumulated. Compound interest is like a snowball effect. In the snowball effect, a snowball size increases when more snow is added. Likewise, in compound interest, one earns interest on the initial principal and also interest on interest. C.I. denotes compound interest in mathematics. It finds its usage in finance and economics.

Introduction

C.I  is the interest generated on a loan or deposit. Its calculation is based on both initial principal and collected interest. C.I is a result of reinvesting interest instead of paying it out. Interest for the next period is earned from the principal sum and previously accumulated interest.

Let’s know what compound interest is. Compound interest is defined as the interest calculated on the principal and the interest accumulated over the previous period of time. Compound interest is different from simple interest. 

In simple interest, the interest is not added to the principal while calculating the interest during the next period while in the compound interest the interest is added to the principal to calculate the interest.

The formula for compound interest is,

where, P is equal to Principal ,  R is equal to Rate of

Interest,  T is equal to Time (Period)

A Little More About Compound Interest

We will first understand the concept and what is compound interest and then move on to the compound interest formula. Now interest can be defined as the amount we calculate on the principal amount that is given to us. But in compound interest, we calculate the interest on the principal amount and the interest that has accumulated during the previous period. Essentially, compound interest is the interest on the interest! So in this method, rather than paying out the interest, it is reinvested and becomes a part of the principal.

As you will have noticed in simple interest, the interest amount remains the same for every period. This is not the case in compound interest. Since the previous interest amount is reinvested, the interest amount increases marginally every year. This is why we have a whole separate compound interest formula to help us calculate the compound interest of any given year. 

The compound interest formula in maths is:

Where, P is equal to Principal,  Rate is equal to Rate of

Interest,  n is equal to the time (Period)

 

Compound Interest Formula Derivation

To better our understanding of the concept, let us take a look at the compound interest formula derivation. Here we will take our principal to be Rupee.1/- and work our way towards the interest amounts of each year gradually.

Year 1

• The interest on Rupee 1/- for 1 year is equal to r/100 = i (assumed)

• Interest after Year 1 is equal to Pi

• FV (Final Value) after Year 1 is equal to P + Pi = P(1+i)

Year 2

Year ‘t’

• Final Value (Amount) after year “t” is equal to P(1+i)^t

• Now substituting actual values we get Final Value is equal to ( 1 + R/100)^t

• CI = FV – P is equal to P ( 1 + R/100)^t – P

This is the Compound Interest Formula Derivation

 

Applications of Compound Interest

Some of the applications of compound interest are:

1. Increase in population or decrease in population.

2. Growth of bacteria.

3. Rise in the value of an item.

4. Depreciation in the value of an item.

Now that we have some clarity about the concept and meaning of compound interest and compound interest formulae in maths, let us try some Compound interest problems with solutions to deepen our understanding of the subject.

 

Compound Interest Problems with Solutions

Q1) The count of a certain breed of bacteria was found to increase at the rate of 5% per hour. What will be the growth of bacteria at the end of 3 hours if the count was initially 6000?

Solution) Since the population of bacteria increases at the rate of 5% per hour,

We know the formula for calculating the amount, compound interest formula in maths 

Amount= Principal(1 + R/100)ⁿ

Thus, the population at the end of 3 hours = 6000(1 + 3/100)³

= 6000(1 + 0.03)³

= 6000(1.03)³

= Rs 6556.36

 

Q2) Mr. A decided to open a bank account and opted for the Compound Interest Option at 10%. He invested 10,000 for 3 years. At the end of three years, how much money will he get, and what will be the interest amount. The interest is calculated annually.

Solution)  As we already have a formula for future value amount, let us substitute the values in the compound interest formula in maths. 

FV = ( 1 + R/100)^t

FV = 10000 ( 1 + 10/100)⁵

FV = 10000 ( 1.1)⁵

FV = 16,105

CI = FV – P = 6,105/-

 

Q3) Mr. B lent money to his son at 8% CI calculated semi-annually. If he lent 1000/- for 2 years, how much will he get back at the end of the 2 years?

Solution) Since the CI is calculated semi-annually

t = 2t = 4

r = r/2 = 4

Final Value = P ( 1 + R/100)^t

FV = 1000 ( 1 + 4/100)⁴

FV = 1000 (1.17)

FV or Amount = 1170/-

Q4. A town will have 10,000 residents in 2020. Its population reduces by 10% per annum. What would the population be in 2025?

Since the population of the town decreases by 10% per annum, it also experiences a new population surge every year.

The population of the upcoming year is calculated on the basis of the current year’s population. So students need to use this formula.

A=P(1-r/100)ⁿ

Henceforth the population at the end of 5 years would be

10000(1-10/100)⁵

10000(1-0.1)⁵

5904.9

A= 5904

C.I Formula

As discussed, C. I findings are based on the initial principal amount and interest over a period of time. The compound interest formula is

C.I= Amount- Principal

Another formula for C.I 

In this,

CI=P(1+r/n)^nt-P

A=  Amount

P= Principal

r=Rate of interest

n= number of interest compounded per year and the compounding frequency

t= time

The above formula is used for a number of times principal compounded in a year. For interest compounded annually, the amount is found through:

A=P(1+R/100)^t

Evaluating the formula for the amount and interest calculation for different years

1 Year

Amount

P(1 + R/100)

Interest

PR/100

2 Year

Amount

P(1+R/100)²

Interest

3 Year

Amount

P(1+R/100)³

Interest

P(1+R/100)³-P

4 year

Amount

P(1+R/100)⁴

Interest

P(1+R/100)⁴-P

N year

Amount

P(1+R/100)ⁿ

Interest

P(1+R/100)ⁿ-P

Derivation of CI

To derive CI, students have to use simple interest formula. This is because SI for 1 year is equal to CI of 1 year.

Let’s assume P as the principal amount, n the time and rate be R.

SI for 1st year

SI1=PxRxT/100

Amount for 1st year=P+ SI

A=P+ PxRxT/100

P(1+R/100)=P₂

SI for 2nd year

SI₂=P₂xRxT/100

Amount for 2nd year=P₂+SI₂

P₂+P₂xRxT/100

 P₂(1 + R/100)

 

P(1 + R/100)(1 + R/100)

 

For n years

A=P(1+R/100)ⁿ

CI=A-P=P((1+R/100)ⁿ-1))

Applications of CI

• Compound interest finds its usage in different phases of life. Some of the common applications of the compound interest formula are as follows:

• Helps in analyzing the growth and decay of the population

• Finding the increase and decrease in commodity price

• Finding the increase and decrease in the value of an item

• Evaluation of inflation in profit and loss

• Bank transactions