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Algebraic Expressions

    Finding the Value of an Expression

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7th class > Algebraic Expressions > Finding the Value of an Expression

Finding the Value of an Expression

We know that the value of an algebraic expression depends on the values of the variables forming the expression. There are a number of situations in which we need to find the value of an expression, such as when we wish to check whether a particular value of a variable satisfies a given equation or not.

We find values of expressions, also, when we use formulas from geometry and from everyday mathematics. For example, the area of a square is l2 where l is the length of a side of the square. If l = 5 cm., the area is 52 cm2 or 25 cm2; if the side is 10 cm, the area is 102 cm2 or 100cm2 and so on. We shall see more such examples.

Example 4

Find the values of the following expressions for x = 2.

19−5x2

  • Inserting x = 2, we get
  • Substituting the value
  • We have found the answer

x+4

  • Inserting x = 2, we get
  • Substituting the value
  • We have found the answer

100−10x3

  • Inserting x = 2, we get
  • Substituting the value
  • We have found the answer

4x−3

  • Inserting x = 2, we get
  • Substituting the value
  • We have found the answer

Example 5

Find the values of the following expressions when n = – 2.

(i) 5n2 + 5n – 2

5n2+5n−2

  • Inserting n = -2, we get
  • Substituting the value
  • We have found the answer.

n3+5n2+5n−2

  • Inserting n = -2, we get
  • Substituting the value
  • We have found the answer

We shall now consider expressions of two variables, for example, x + y, xy.

To work out the numerical value of an expression of two variables, we need to give the values of both variables. For example, the value of (x + y), for x = 3 and y = 5, is 3 + 5 = .

Example 6

Find the value of the following expressions for a = 3, b = 2.

(i) a3-b3

a3−b3

  • Inserting a = 3 and b = 2, we get
  • Substituting the value
  • We have found the answer

a2+2ab+b2

  • Inserting a = 3 and b = 2, we get
  • Substituting the value
  • We have found the answer