Determine X for Negative Values of the Function: y=2x²-24x

Quadratic Inequalities with Interval Testing

Look at the following function:

y=2x224x y=2x^2-24x

Determine for which values of x x the following is true:

f(x)<0 f\left(x\right) < 0

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Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Look at the following function:

y=2x224x y=2x^2-24x

Determine for which values of x x the following is true:

f(x)<0 f\left(x\right) < 0

2

Step-by-step solution

To solve the problem, follow these steps:

  • Step 1: Solve the quadratic equation 2x224x=0 2x^2 - 24x = 0 .
  • Step 2: Factor the equation as 2x(x12)=0 2x(x - 12) = 0 .
  • Step 3: Determine the roots, which are x=0 x = 0 and x=12 x = 12 .
  • Step 4: Analyze the intervals determined by these roots: x<0 x < 0 , 0<x<12 0 < x < 12 , and x>12 x > 12 .
  • Step 5: Test each interval to see where f(x)<0 f(x) < 0 .

Now let's work through each step:

Step 1: Solve the equation 2x(x12)=0 2x(x - 12) = 0 . This gives us roots at x=0 x = 0 and x=12 x = 12 .

Step 2: The quadratic can be negative between the roots, so we consider the interval 0<x<12 0 < x < 12 .

Step 3: Test the sign of 2x(x12) 2x(x - 12) in each interval:

  • Interval (,0) (-\infty, 0) : Choose a test point x=1 x = -1 . The expression is 2(1)((1)12)=2(1)(13)=26 2(-1)((-1) - 12) = 2(-1)(-13) = 26 , which is positive.
  • Interval (0,12) (0, 12) : Choose a test point x=1 x = 1 . The expression is 2(1)(112)=2(1)(11)=22 2(1)(1 - 12) = 2(1)(-11) = -22 , which is negative.
  • Interval (12,) (12, \infty) : Choose a test point x=13 x = 13 . The expression is 2(13)(1312)=2(13)(1)=26 2(13)(13 - 12) = 2(13)(1) = 26 , which is positive.

Thus, the quadratic is negative in the interval 0<x<12 0 < x < 12 .

Therefore, the solution to the problem is 0<x<12 0 < x < 12 .

3

Final Answer

0<x<12 0 < x < 12

Key Points to Remember

Essential concepts to master this topic
  • Rule: Find roots first, then test intervals between them
  • Technique: Factor 2x224x=2x(x12) 2x^2 - 24x = 2x(x - 12) to get roots x = 0, 12
  • Check: Test x = 1: 2(1)(112)=22<0 2(1)(1-12) = -22 < 0

Common Mistakes

Avoid these frequent errors
  • Forgetting to test intervals between roots
    Don't just find the roots x = 0 and x = 12 and guess the answer = wrong solution! The sign of a quadratic changes at each root, so you must test each interval separately. Always pick test points in each interval and evaluate the factored form.

Practice Quiz

Test your knowledge with interactive questions

The graph of the function below does not intersect the \( x \)-axis.

The parabola's vertex is marked A.

Find all values of \( x \) where
\( f\left(x\right) > 0 \).

AAAX

FAQ

Everything you need to know about this question

Why do I need to factor the quadratic first?

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Factoring makes it much easier to find the roots and test intervals! The factored form 2x(x12) 2x(x-12) clearly shows the roots are x = 0 and x = 12.

How do I know which intervals to test?

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The roots divide the number line into intervals. With roots at 0 and 12, test the three intervals: x<0 x < 0 , 0<x<12 0 < x < 12 , and x>12 x > 12 .

What test points should I choose?

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Pick any convenient number from each interval! For x<0 x < 0 try x = -1, for 0<x<12 0 < x < 12 try x = 1, and for x>12 x > 12 try x = 13.

Why is the quadratic negative between the roots?

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This quadratic opens upward (positive leading coefficient). It starts positive, crosses zero at the roots, goes negative between them, then becomes positive again after the second root.

Do I include the roots x = 0 and x = 12 in my answer?

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No! The problem asks for f(x)<0 f(x) < 0 , which means strictly less than zero. At the roots, f(x)=0 f(x) = 0 , not negative.

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