Solve: Finding Positive Values for y = -1/6x² - 3⅓x

Quadratic Inequalities with Factored Form

Look at the following function:

y=16x2319x y=-\frac{1}{6}x^2-3\frac{1}{9}x

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

f(x)>0 f(x) > 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=16x2319x y=-\frac{1}{6}x^2-3\frac{1}{9}x

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

f(x)>0 f(x) > 0

2

Step-by-step solution

To solve this problem, we'll follow these steps:

  • Step 1: Identify the given equation in a standard form.
  • Step 2: Calculate the roots using the quadratic formula.
  • Step 3: Determine the sign of the function in different intervals determined by the roots.

Now, let's work through each step:

Step 1: The given quadratic function is y=16x2103x y = -\frac{1}{6}x^2 - \frac{10}{3}x . Here, a=16 a = -\frac{1}{6} , b=103 b = -\frac{10}{3} , and c=0 c = 0 .
We rewrite the function as y=16x2103x y = -\frac{1}{6}x^2 - \frac{10}{3}x .

Step 2: To find the roots of the quadratic equation, apply the quadratic formula:
x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}

Plugging in the values, we get:
x=(103)±(103)24(16)02(16) x = \frac{-(-\frac{10}{3}) \pm \sqrt{(-\frac{10}{3})^2 - 4 \cdot (-\frac{1}{6}) \cdot 0}}{2 \cdot (-\frac{1}{6})}
This simplifies to:
x=10313 x = \frac{\frac{10}{3}}{-\frac{1}{3}} (since the discriminant simplifies to zero as c c is zero)
x=0 x = 0 and x=1823 x = -18\frac{2}{3} (solving for the roots)

Step 3: Determining the intervals:
Because the parabola opens downwards (as a=16 a = -\frac{1}{6} is negative), the quadratic is positive between the roots.
Thus, f(x)>0 f(x) > 0 in the interval 1823<x<0 -18\frac{2}{3} < x < 0 .

Therefore, the solution to the problem is 1823<x<0 -18\frac{2}{3} < x < 0 .

3

Final Answer

1823<x<0 -18\frac{2}{3} < x < 0

Key Points to Remember

Essential concepts to master this topic
  • Rule: Find roots first, then test intervals between them
  • Technique: Factor out x: x(16x103)=0 x(-\frac{1}{6}x - \frac{10}{3}) = 0 gives roots 0 and 1823 -18\frac{2}{3}
  • Check: Test x = -10: 16(10)2103(10)=703>0 -\frac{1}{6}(-10)^2 - \frac{10}{3}(-10) = \frac{70}{3} > 0

Common Mistakes

Avoid these frequent errors
  • Confusing when parabola is positive vs negative
    Don't assume f(x) > 0 outside the roots for all parabolas = wrong intervals! This ignores whether the parabola opens up or down. Always check: negative coefficient of x² means parabola opens downward, so f(x) > 0 between the roots.

Practice Quiz

Test your knowledge with interactive questions

The graph of the function below intersects the X-axis at points A and B.

The vertex of the parabola is marked at point C.

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

AAABBBCCCX

FAQ

Everything you need to know about this question

How do I know which direction the parabola opens?

+

Look at the coefficient of x2 x^2 ! If it's positive, the parabola opens upward (U-shape). If it's negative (like 16 -\frac{1}{6} here), it opens downward (∩-shape).

Why is the function positive between the roots?

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Since our parabola opens downward, it's like an upside-down U. The function is positive (above the x-axis) between the two roots where it crosses zero, and negative outside the roots.

What if I can't factor the quadratic easily?

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Use the quadratic formula to find the roots: x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} . Here, we can factor out x first since there's no constant term!

How do I convert the mixed number in the answer?

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1823=563 18\frac{2}{3} = \frac{56}{3} because 18×3+2=56 18 \times 3 + 2 = 56 . So 1823=563 -18\frac{2}{3} = -\frac{56}{3} .

Should I test points to verify my interval?

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Yes! Pick any value inside your interval (like x = -10) and substitute it into the original function. If you get a positive result, your interval is correct!

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