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Equations with Infinite Solutions and Equations with No Solution
> ... Math > Solving for a Variable > Equations with Infinite Solutions and Equations with No Solution

Not all equations we try to solve will end with \(x\) = a specific number. Some equations may have infinitely many solutions and other equations may have no solution at all. The following video will show how to recognize these solutions.

Special Cases Equations with Infinite Solutions and Equations with No Solutions

Video Source (05:35 mins) | Transcript

There are three types of answers we can get when solving for a variable:

  1. \(x\) = A specific number (this is what we’ve been getting until now such as \( x = 5.3\) )
  2. \(x\) = All real numbers or infinitely many solutions (when we get x=x or when any number is equal to itself such as \(3 = 3\) )
  3. No Solutions (when we end with a false statement like \( 1 = 5 \) )

Additional Resources

Practice Problems

  1. \(-9{\text{M}} {-} 4 = -9{\text{M}} - 4\) (
    Solution
    x
    Solution: Infinitely Many Solutions
    Details:
    In this example, the first thing we need to do is combine like terms. This means we combine the terms with the variable \(M\) with each other and we combine the terms without a variable together.

    Note: There are two versions or ways to solve this equation. Either one is acceptable. You do not have to do both.

    First version: Combine terms with variable M first

    There is currently a \(−9M\) on the right-hand side of the equation. We can remove it from the right-hand side and combine it with the left-hand side by adding \(+9M\) to both sides of the equation.

    This image shows the equation negative nine M minus four equals negative nine M minus four. Under the negative nine M on both sides of the equal sign is a green positive nine M.

    On the right-hand side:

    \(-9M + 9M = 0\) leaving just \(-4\)

    This image shows the equation negative nine M minus four equals negative nine M minus four. Under the negative nine M on both sides of the equal sign is a positive nine M. There is a light gray dotted line running vertically through the equal sign. Below the previous material is a solid horizontal line. Below the line are the results of the operations, but only for the right-hand side. A blue arrow at the very top of the right-hand side indicates we are working with the operations on this side. Below the addition line is a blue zero which is the result of negative nine M plus positive nine M. There is also minus four.

    On the left-hand side:

    \(-9M + 9M = 0\) leaving just \(-4\)

    This image shows the equation negative nine M minus four equals negative nine M minus four. Under the negative nine M on both sides of the equal sign is a positive nine M. There is a light gray dotted line running vertically through the equal sign. Below the previous material is a solid line, like under an addition problem. Below the line are the results of the operations. A blue arrow at the very top of the left-hand side indicates we are working with the operations on this side. Below the addition line on the left-hand side is a blue zero which is the result of negative nine M plus positive nine M. There is also minus four. This is all equal to zero minus 4 on the right-hand side too.

    Because \(−9M+9M=0\), we are left with \(−4=−4\). This statement is always true, therefore, there are infinitely many solutions for the equation \(−9M−4=−9M−4\). This means that any value of M will still make this equation true

    Our final solution: Infinitely many solutions

    Second version: Combine terms without a variable first

    We want to combine the \(−4\) on the left-hand side of the equation with the \(−4\) on the right-hand side of the equation. To do this, add \(+4\) to both sides of the equation.

    This image shows the equation negative nine M minus four equals negative nine M minus four. There is a positive four under the minus four on both sides of the equal sign. A blue arrow above the minus four and positive four indicates we are working with these terms.

    On the left-hand side:

    \(−4+4=0\)

    On the right-hand side:

    \(−4+4=0\)

    This image shows the equation negative nine M minus four equals negative nine M minus four. There is a positive four under the minus four on both sides of the equal sign. Below all of this is a horizontal black line and under it is a new equation, the result of the operations above it. This bottom equation is negative nine M plus zero equals negative nine M plus zero. The plus zero on both sides is in blue, indicating it is the part of the equation that changed from the original due to the addition of positive four to both sides.

    This leaves us with \(−9M=−9M\). We can either stop here because we see both sides are equal to each other, which means that for any value of M the statement will be true, or we can keep solving for \(M\).

    To keep solving for \(M\), we need to multiply both sides by the multiplicative inverse of \(−9\). Multiply both sides by \(-\dfrac{1}{9}\).

    This image is (negative one-ninth) multiplied to (negative nine M) equals (negative one-ninth) multiplied to (negative nine M). The fractions one-ninth on both sides of the equal sign are green, indicating they are new.

    \(\left (-\dfrac{1}{9} \right )\left ( -9 \right )=1\)

    This leaves \(1M=1M\).

    This image has three equations listed one above the other. The first equation is (negative one-ninth) multiplied to (negative nine M) equals (negative one-ninth) multiplied to (negative nine M). The fractions one-ninth on both sides of the equal sign are green. The equation below this is the same except the negative 9 is separated by parentheses from the M. (Negative one-ninth) multiplied to (negative nine) multiplied to M equals (negative one-ninth) multiplied to (negative nine) multiplied to M. Below the (negative one-ninth) multiplied to (negative nine) there is a blue bracket grouping them together and the point of the blue bracket points to blue text that says, “Multiplicative inverses =1.” This is the same on both sides of the equal sign. Below all of this s the last equation which is a blue number 1 multiplied to M equals a blue number 1 multiplied to M.

    \(M=M\) is always true for any value of \(M\).

    Our final solution: Infinitely Many Solutions
    )
  2. \(9 + 8{\text{T}} = 13{\text{T}} + 2\) (
    Solution
    x
    Solution:
    One Solution
    )
  3. \(-4 + 2{\text{b}} = 2{\text{b}} - 9\) (
    Solution
    x
    Solution: No Solution
    Details:
    We start by combining like terms.

    Combine the terms with the variable b by adding \(−2b\) to both sides of the equation.

    This image shows the equation negative four plus two b equals two b minus nine. Under the two b on both sides of the equal sign is written plus negative two b. The negative two b is in red indicating it is new.

    Since \(2b+(−2b)=0\), we are left with \(−4\) on the left-hand side and \(−9\) on the right-hand side.

    This image shows the equation negative four plus two b equals two b minus nine. Under the two b on both sides of the equal sign is written plus negative two b. The negative two b is in red indicating it is new. Below all of this is a long horizontal line. Below it are the results of the operations above. Directly below the line is the equation negative four plus zero equals zero minus nine. The zero on both sides is written in green to indicate it is the result of the addition above it. Under all of this is the new equation negative four equals negative nine.

    But \(−4\) does not equal \(−9\).

    This means that no matter what values we put into this equation, it is not true.

    Our final solution: No solution
    )
  4. \(-7 + 7{\text{b}} + 18 = 3{\text{b}} + 3 - 4{\text{b}}\) (
    Solution
    x
    Solution:
    One Solution
    )
  5. \(2{\text{x}} + 5 + {\text{x}} = -1 + 3{\text{x}} + 6\) (
    Solution
    x
    Solution:
    Infinitely Many Solutions
    )
  6. \(2(3{\text{X}} + 4) = 6{\text{X}} + 7\) (
    Video Solution
    x
    Solution: No Solution
    Details:

    (Video Source | Transcript)
    )
  7. \(-4(4{\text{M}} {-} 3) = -16{\text{M}} + 12\) (
    Video Solution
    x
    Solution: Infinitely Many Solutions
    Details:

    (Video Source | Transcript)
    )

Need More Help?

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Solving Variables on Both Sides of the Equation
Substitute Values into an Equation and Solve for a Variable