In algebra we will learn how to find the value of the variables by changing the subject in an equation or formula using the balance method.

To find the value of the variable using change of subject of formula and method of substitution:

The process of finding the value of algebraic expression by replacing the variables by the given value is called the **method of substitution**.

**In this,**

**•** Make the required variable as the subject.

**•** Substitute the values of all the variables except the subject.

**•** Now simplify and find the value of the subject.

**Worked-Out Examples on Changing the Subject in an Equation or Formula**

**1.** In the formula **P = 2(l + b)**, find **b** when **P** = 36 cm and **l** = 10 m

**Solution:**

P = 2 (l + b)

P = 2l + 2b

P - 2l = 2b

(P - 2l)/2 = 2̶b/2̶

i.e., b = (P - 2l)/2

b = (36 - 2 × 10)/2

b = (36 - 20)/2

b = 1̶6̶/2̶

b = 8

**2.** Given A = P {1 + (r/100)}ⁿ make **r** as the subject.

Given A = 1102.50 P = 1000 n = 2, find **r**.

**Solution:**

A = P {1 + (r/100)}ⁿ

A/P = {1 + (r/100)}ⁿ

(A/P)

100 {(A/P)

Therefore, 100 {(A/P)

r = 100 {(1102.50/1000)

= 100 {(110250/1000)

= 100 {(441/400)

= 100 [{(21/20)

= 100 {(21/20)

= 100 {21/20 - 1}

= 100 {(21 – 20)/20}

= 1̶0̶0̶ × 1/2̶0̶

= 5

**● **Formula

**Formula and Framing the Formula**

**Change the Subject of a Formula**

**Changing the Subject in an Equation or Formula**

**Practice Test on Framing the Formula**

● Formula - Worksheets

**Worksheet on Framing the Formula**

**Worksheet on Changing the Subject of a Formula**

**Worksheet on Changing the Subject in an Equation or Formula**

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