Domain Analysis: Solve (x+1/6)(-x-4/9) > 0 Step by Step

Quadratic Inequalities with Mixed Numbers

Find the positive and negative domains of the function below:

y=(x+16)(x419) y=\left(x+\frac{1}{6}\right)\left(-x-4\frac{1}{9}\right)

Then 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

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1

Understand the problem

Find the positive and negative domains of the function below:

y=(x+16)(x419) y=\left(x+\frac{1}{6}\right)\left(-x-4\frac{1}{9}\right)

Then 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 this problem, we will determine where the function, given as y=(x+16)(x419) y = \left(x + \frac{1}{6}\right)\left(-x - 4\frac{1}{9}\right) , is positive.

Step 1: **Find the Roots**
Set the function equal to zero: (x+16)(x419)=0 \left(x + \frac{1}{6}\right)\left(-x - 4\frac{1}{9}\right) = 0 . This yields:
- x+16=0 x + \frac{1}{6} = 0 which gives x=16 x = -\frac{1}{6} , and
- x419=0 -x - 4\frac{1}{9} = 0 which gives x=419 x = -4\frac{1}{9} .
Thus, the roots are x=16 x = -\frac{1}{6} and x=419 x = -4\frac{1}{9} .

Step 2: **Analyze Sign Intervals**
The parabola opens downwards because the product has a negative coefficient as the leading term.
We have intervals: x<419 x < -4\frac{1}{9} , 419<x<16 -4\frac{1}{9} < x < -\frac{1}{6} , and x>16 x > -\frac{1}{6} .

Since the quadratic opens downwards, it is positive between the roots (-4\frac{1}{9}, -\frac{1}{6}), where f(x)>0 f(x) > 0 .

Therefore, the solution for the values of x x for which f(x)>0 f(x) > 0 is:

419<x<16 -4\frac{1}{9} < x < -\frac{1}{6}

3

Final Answer

419<x<16 -4\frac{1}{9} < x < -\frac{1}{6}

Key Points to Remember

Essential concepts to master this topic
  • Factored Form: Find zeros by setting each factor equal to zero
  • Sign Analysis: Test intervals between roots: 419<x<16 -4\frac{1}{9} < x < -\frac{1}{6}
  • Verification: Pick test point x = -0.2 and confirm f(-0.2) > 0 ✓

Common Mistakes

Avoid these frequent errors
  • Converting mixed numbers incorrectly
    Don't write 4⅑ as 4.1 or forget the negative sign = wrong roots! Mixed number -4⅑ equals -37/9, not -4.1. Always convert mixed numbers to improper fractions: -4⅑ = -37/9.

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

Why does the parabola open downward?

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The second factor (-x - 4⅑) has a negative coefficient for x. When expanded, this gives a negative coefficient for the x2 x^2 term, making the parabola open downward.

How do I know which interval makes the function positive?

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Since the parabola opens downward, it's positive between the roots and negative outside them. Test a point like x = -0.2 (which is between -4⅑ and -⅙) to confirm.

What's the difference between -4⅑ and -⅙?

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Convert to decimals: 4194.11 -4\frac{1}{9} ≈ -4.11 and 160.17 -\frac{1}{6} ≈ -0.17 . So -4⅑ is much smaller (more negative) than -⅙.

Why don't we include the roots in our answer?

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The inequality asks for f(x) > 0, which means strictly greater than zero. At the roots x = -4⅑ and x = -⅙, the function equals zero, not greater than zero.

How can I check my interval is correct?

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Pick any number in your interval and substitute it. For example, try x = -0.3: (0.3+16)((0.3)419)>0 (-0.3 + \frac{1}{6})(-(-0.3) - 4\frac{1}{9}) > 0 should be true.

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