Solve the Quadratic Inequality: -x² + 2x > 0

Quadratic Inequalities with Factoring Method

Solve the following equation:

x2+2x>0 -x^2+2x>0

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1

Understand the problem

Solve the following equation:

x2+2x>0 -x^2+2x>0

2

Step-by-step solution

To solve the inequality x2+2x>0-x^2 + 2x > 0, we begin by considering the corresponding equation x2+2x=0-x^2 + 2x = 0.

First, factor the quadratic equation:

  • Rearrange the terms: x2+2x=0-x^2 + 2x = 0 becomes x(2x)=0x(2 - x) = 0.
  • This gives us the roots x=0x = 0 and x=2x = 2.

These roots divide the number line into three intervals: x<0x < 0, 0<x<20 < x < 2, and x>2x > 2.

We need to test these intervals to determine where the inequality holds:

  • For x<0x < 0, choose a test point like x=1x = -1: the expression (1)2+2(1)=12=3-(-1)^2 + 2(-1) = -1 - 2 = -3, which is not greater than zero.
  • For 0<x<20 < x < 2, choose a test point like x=1x = 1: the expression (1)2+2(1)=1+2=1-(1)^2 + 2(1) = -1 + 2 = 1, which is greater than zero.
  • For x>2x > 2, choose a test point like x=3x = 3: the expression (3)2+2(3)=9+6=3-(3)^2 + 2(3) = -9 + 6 = -3, which is not greater than zero.

Thus, the inequality x2+2x>0-x^2 + 2x > 0 is satisfied for the interval 0<x<20 < x < 2.

Therefore, the solution to the inequality is 0<x<2\mathbf{0 < x < 2}, which corresponds to choice 2 in the given options.

3

Final Answer

0<x<2 0 < x < 2

Key Points to Remember

Essential concepts to master this topic
  • Rule: Factor the quadratic and find roots to identify critical points
  • Technique: Test intervals between roots: x=1 x = 1 gives 1+2=1>0 -1 + 2 = 1 > 0
  • Check: Verify boundary points are excluded since inequality is strictly greater than zero ✓

Common Mistakes

Avoid these frequent errors
  • Including the boundary points in the solution
    Don't write 0x2 0 \leq x \leq 2 when solving x2+2x>0 -x^2 + 2x > 0 = incorrect solution! The inequality is strictly greater than zero, so x=0 x = 0 and x=2 x = 2 make the expression equal zero, not greater. Always use open intervals for strict inequalities.

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 the quadratic first?

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Factoring helps you find the roots where the expression equals zero. These roots are the boundary points that divide the number line into intervals where the inequality might change from true to false.

How do I know which intervals to test?

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The roots x=0 x = 0 and x=2 x = 2 create three regions: before 0, between 0 and 2, and after 2. Pick any test point from each region to see where the inequality holds.

What's the difference between > and ≥ in the final answer?

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Since our inequality is x2+2x>0 -x^2 + 2x > 0 (strictly greater), we use open intervals like 0<x<2 0 < x < 2 . If it were ≥, we'd include the boundary points where the expression equals zero.

Can I solve this by graphing instead?

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Yes! Graph y=x2+2x y = -x^2 + 2x and look for where the parabola is above the x-axis. The solution is the x-values where y>0 y > 0 .

Why is my test point calculation giving a negative result?

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Check your arithmetic carefully! For x=1 x = 1 : 12+2(1)=1+2=1 -1^2 + 2(1) = -1 + 2 = 1 , which is positive. Remember that 12=1 -1^2 = -1 , not +1 +1 .

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