Solve (3x+3)(2-x) < 0: Finding Values Where Function is Negative

Quadratic Inequalities with Factored Form

Look at the following function:

y=(3x+3)(2x) y=\left(3x+3\right)\left(2-x\right)

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=(3x+3)(2x) y=\left(3x+3\right)\left(2-x\right)

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 identify the range of x x such that y=(3x+3)(2x)<0 y = (3x + 3)(2 - x) < 0 , we'll follow these steps:

  • Step 1: Find the roots of the equation by solving (3x+3)=0 (3x + 3) = 0 and (2x)=0 (2 - x) = 0 .
  • Step 2: Find the intervals created by these roots.
  • Step 3: Test each interval to determine the sign of the product.

Let's execute each step:

Step 1: Solving the equations:
First root: Set 3x+3=0 3x + 3 = 0 which gives x=1 x = -1 .
Second root: Set 2x=0 2 - x = 0 which gives x=2 x = 2 .

Step 2: The roots divide the real number line into three intervals:

  • x<1 x < -1
  • 1<x<2 -1 < x < 2
  • x>2 x > 2

Step 3: Analyze each interval:

- For x<1 x < -1 : Choose x=2 x = -2 . The expression becomes (3(2)+3)(2(2))=(3)(4)=12 (3(-2) + 3)(2 - (-2)) = (-3)(4) = -12 , which is negative.

- For 1<x<2 -1 < x < 2 : Choose x=0 x = 0 . The expression becomes (3(0)+3)(20)=(3)(2)=6 (3(0) + 3)(2 - 0) = (3)(2) = 6 , which is positive.

- For x>2 x > 2 : Choose x=3 x = 3 . The expression becomes (3(3)+3)(23)=(12)(1)=12 (3(3) + 3)(2 - 3) = (12)(-1) = -12 , which is negative.

Therefore, the function is negative for x<1 x < -1 or x>2 x > 2 .

The solution to this problem is x>2 x > 2 or x<1 x < -1 .

3

Final Answer

x>2 x > 2 or x<1 x < -1

Key Points to Remember

Essential concepts to master this topic
  • Roots: Set each factor equal to zero to find boundary points
  • Sign Analysis: Test values in each interval: (-2) gives (-3)(4) = -12
  • Verification: Function changes sign at x = -1 and x = 2 ✓

Common Mistakes

Avoid these frequent errors
  • Solving the inequality like an equation
    Don't just solve (3x+3)(2-x) = 0 and call it done = missing the actual solution intervals! This only gives you the boundary points, not where the function is negative. Always test intervals between roots to determine where the inequality holds.

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 find the roots first?

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The roots are where the function equals zero, which creates the boundary points. These divide the number line into intervals where the function stays either positive or negative without changing sign.

How do I know which intervals to test?

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Once you find the roots, they create intervals on the number line. Test one value from each interval by substituting it into the original expression to see if it's positive or negative.

What if I get the wrong sign when testing?

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Double-check your arithmetic! Make sure you're substituting correctly: for x=2 x = -2 , you get (3(2)+3)(2(2))=(3)(4)=12 (3(-2)+3)(2-(-2)) = (-3)(4) = -12 which is negative.

Why isn't the answer just x = -1 or x = 2?

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Those values make the function equal to zero, not less than zero! We want f(x)<0 f(x) < 0 , so we need the intervals where the function is negative: x<1 x < -1 or x>2 x > 2 .

Can I expand the expression first instead?

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You could, but it's much harder! The factored form (3x+3)(2x) (3x+3)(2-x) makes it easy to find roots and analyze signs. Keep it factored whenever possible.

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