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Math Lesson 9.3.3 - Identify Conditional Identities and Inconsistent Equations

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Welcome to our Math lesson on Identify Conditional Identities and Inconsistent Equations, this is the third lesson of our suite of math lessons covering the topic of Identities, you can find links to the other lessons within this tutorial and access additional Math learning resources below this lesson.

How to Distinguish Identities from Conditional Identities and Inconsistent Equations?

It is not appropriate to substitute values in a given mathematical sentence to check whether it is an identity, conditional or inconsistent equation because it may give a true result for some values and you may think it is an identity but there are other values that you have not considered which make the sentence false. Therefore, it is better to find a general form of all these three types of mathematical sentences; then check which category the given sentence does belong to.

From above, it is clear that the general form of identities (after having made all possible operations) is

0xn = 0

where n is the order (highest power) of the original equation. In first - order identities, we have n = 1, in second - order ones n = 2 and so on.

In other words, all variables, coefficients and constants become zero after all operations and simplification are made. For example, in the equation

2 - 3x = 4x - 1 - 7x + 3

we have

2 - 3x = 4x - 1 - 7x + 3
2 - 3x = - 3x + 2

Adding -3x and 2 to both sides to remove all terms from the right side yields

2 - 3x + 3x - 2 = - 3x + 3x + 2 - 2
0x = 0

Hence, this is an identity.

On the other hand, if we have an equation that, after having made all possible operations and simplifications, takes the form

ax = b (or ax - b = 0)

where a and b are numbers (a is a coefficient and b is a constant) and b is a variable, then we are dealing with a conditional identity, which is nothing more than a normal equation, like those we discussed in prior tutorials. Such equations have a limited maximum number of solutions depending on their order and number of variables. Thus, the first - order equations with one variable have at maximum one solution, the second - order equations with one variable (quadratic equations) have two solutions at maximum, and so on.

For example, 7x - 21 = 0 has a single solution (x = 3), because 7 · 3 - 21 = 0. All the other numbers give false results in regard to the equality between the two sides of the equation.

Last, if we have an equation that after having made all possible operations and simplifications gives a general form

0x = b

where b is a constant, then we are dealing with an inconsistent equation, i.e. with an equation that has no solutions.

For example, 3x - 4 = - 2x + 1 + 5x is an inconsistent equation as after doing the operations, we obtain

3x - 4 = - 2x + 1 + 5x
3x - 4 = 3x + 1
3x - 3x - 4 - 1 = 3x - 3x + 1 - 1
0x - 5 = 0x + 0
0x = 5

(Here, b = 5)

Example 1

Find whether the following mathematical sentences are identities, conditional identities (equations) or inconsistent equations.

  1. (2x - 1)2 = 1 - 4x + 4x2
  2. 4 - 3x = 3 - 3(x + 2)

Solution 1

  1. When the left side of this equation expands, it has the same expression as the one on the right side as it represents the square of a difference (the second special algebraic identity discussed in tutorial 6.3). This indicates that we are dealing with an identity. Indeed,
    (2x - 1)2
    = (2x)2 - 2 ∙ (2x) ∙ 1 + 12
    = 4x2 - 4x + 1
    This is identical to the right side of the first equation, as based on the commutative property of addition, we have
    1 - 4x + 4x2
    = 4x2 - 4x + 1
    Therefore, we have
    4x2 - 4x + 1 = 4x2 - 4x + 1
    Sending all terms on the left side yields
    4x2 - 4x2 - 4x + 4x + 1 - 1 = 0
    0x2 + 0x + 0 = 0
    0x2 = 0
    which represents the general form of identity where n = 2.
  2. Solving this equation yields
    4 - 3x = 3 - 3(x + 2)
    4 - 3x = 3 - 3 ∙ x - 3 ∙ 2
    4 - 3x = 3 - 3x - 6
    4 - 3x = - 3 - 3x
    Sending variables on the left and numbers on the right yields
    - 3x + 3x = - 3 - 4
    0x = - 7
    This is an inconsistent equation, as it has the general form 0x = b, where b = -7.

More Identities Lessons and Learning Resources

Equations Learning Material
Tutorial IDMath Tutorial TitleTutorialVideo
Tutorial
Revision
Notes
Revision
Questions
9.3Identities
Lesson IDMath Lesson TitleLessonVideo
Lesson
9.3.1Overview
9.3.2Identities, Conditional Identities (Equations) and Inconsistent Equations
9.3.3Identify Conditional Identities and Inconsistent Equations
9.3.4Eight Algebraic Identities
9.3.5Solving by Proof
9.3.6Rejecting a Supposition by Counter Example

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