Solve the Quadratic Inequality: Finding When x²+6x>0

Quadratic Inequalities with Factoring Method

Solve the following equation:

x2+6x>0 x^2+6x>0

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1

Understand the problem

Solve the following equation:

x2+6x>0 x^2+6x>0

2

Step-by-step solution

To solve the inequality x2+6x>0 x^2 + 6x > 0 , follow these steps:

  • Step 1: Write the inequality in factored form.
    Express x2+6x x^2 + 6x as x(x+6) x(x + 6) .
  • Step 2: Identify the roots of the equation x(x+6)=0 x(x + 6) = 0 .
    The roots are x=0 x = 0 and x=6 x = -6 .
  • Step 3: Determine the sign of x(x+6) x(x + 6) in each interval divided by the roots.
  • Step 4: Test three intervals: x<6 x < -6 , 6<x<0 -6 < x < 0 , and x>0 x > 0 .

For x<6 x < -6 :
Pick a value such as x=7 x = -7 . Substituting, x(x+6)=(7)(7+6)=(7)(1)=7>0 x(x + 6) = (-7)(-7 + 6) = (-7)(-1) = 7 > 0 .
Thus, x2+6x>0 x^2 + 6x > 0 for x<6 x < -6 .

For 6<x<0-6 < x < 0 :
Pick a value such as x=3 x = -3 . Substituting, x(x+6)=(3)(3+6)=(3)(3)=9<0 x(x + 6) = (-3)(-3 + 6) = (-3)(3) = -9 < 0 .
Thus, x2+6x<0 x^2 + 6x < 0 for 6<x<0-6 < x < 0 .

For x>0 x > 0 :
Pick a value such as x=1 x = 1 . Substituting, x(x+6)=(1)(1+6)=1×7=7>0 x(x + 6) = (1)(1 + 6) = 1 \times 7 = 7 > 0 .
Thus, x2+6x>0 x^2 + 6x > 0 for x>0 x > 0 .

Therefore, the solution to the inequality is x<6 x < -6 or x>0 x > 0 .

Thus, the correct answer is x<6,0<x x < -6, 0 < x .

3

Final Answer

x<6,0<x x < -6,0 < x

Key Points to Remember

Essential concepts to master this topic
  • Factoring: Express x2+6x x^2 + 6x as x(x+6) x(x + 6) to find roots
  • Sign Testing: Check intervals using test values like x=7 x = -7 gives positive result
  • Verification: Substitute test values to confirm x<6 x < -6 and x>0 x > 0 work ✓

Common Mistakes

Avoid these frequent errors
  • Solving the inequality like an equation
    Don't just find where x2+6x=0 x^2 + 6x = 0 and think you're done = missing the solution intervals! The roots x=0 x = 0 and x=6 x = -6 are boundary points, not the answer. Always test intervals between roots to find where the inequality is satisfied.

Practice Quiz

Test your knowledge with interactive questions

Solve the following equation:

\( x^2+4>0 \)

FAQ

Everything you need to know about this question

Why do I need to factor first?

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Factoring x2+6x x^2 + 6x into x(x+6) x(x + 6) reveals the roots where the expression equals zero. These roots divide the number line into intervals where you can test the sign of the expression.

How do I know which intervals to test?

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The roots x=6 x = -6 and x=0 x = 0 create three intervals: before -6, between -6 and 0, and after 0. Pick any test value from each interval to check the sign.

What if my test value gives a negative result?

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That means the original expression is negative in that interval. Since we want x2+6x>0 x^2 + 6x > 0 (positive), we exclude intervals where our test gives negative results.

Why isn't the answer just x > 0?

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Because the expression is also positive when x<6 x < -6 ! Testing x=7 x = -7 gives (7)(1)=7>0 (-7)(-1) = 7 > 0 . Always test all intervals, not just the obvious ones.

Do I include the boundary points x = -6 and x = 0?

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No! Since we need x2+6x>0 x^2 + 6x > 0 (strictly greater than), and both boundary points make the expression equal to zero, we exclude them from our solution.

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