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Basic Concept of Substitution

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Topics

Estimated time: 7 minutes
  • Introduction
  • The Substitution Process
  • Example 1
  • Example 2
  • Key Points Summary
CISCE: Class 6

Introduction

Algebraic expressions combine numbers, variables (letters), and operation signs to form mathematical statements. An expression’s value changes when you replace its variables with different numbers—this process is called substitution.

CISCE: Class 6

The Substitution Process

  • Step 1: Identify the variable in the expression
  • Step 2: Replace the variable with the given number
  • Step 3: Follow the order of operations (BODMAS)
  • Step 4: Calculate the final answer
CISCE: Class 6

Example 1

  1. Write the expression:
  2.  Substitute x = 2, the value of the expression 3x + 2 = 3 × 2 + 2 = 6 + 2 = 8

  3. Substitute x = 0, the value of the expression 3x + 2 = 3 × 0 + 2 = 0 + 2 = 2

  4. Substitute  x = −2, the value of the expression 3x + 2 = 3 × (−2) + 2 = −6 + 2 = −4, and so on.
CISCE: Class 6

Example 2

If x = 5, y = 6 and z = 10, find the value of

(i) `(3x^2)/(x)` = `(3×5^2)/(5)` = `"3 × 5 × 5" / 5` =  3 × 5 = 15

 Alternative method:

`(3x^2)/(x)` = `"3 × x × x" / x` = 3 × x = 3 × 5 = 15 

CISCE: Class 6

Key Points Summary

  • Substitution transforms expressions into numbers.

  • Always replace every occurrence of a variable.

  • Follow order of operations (BODMAS).

  • Use parentheses for negative values.

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