Analyze the Quadratic y = 2x² - 4x + 2: Determining Positive and Negative Domains

Quadratic Functions with Discriminant Analysis

Find the positive and negative domains of the function below:

y=2x24x+2 y=2x^2-4x+2

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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=2x24x+2 y=2x^2-4x+2

2

Step-by-step solution

To solve for the positive and negative domains of the function y=2x24x+2 y = 2x^2 - 4x + 2 , we follow these steps:

  • Step 1: Find the roots of the equation using the quadratic formula. For ax2+bx+c=0 ax^2 + bx + c = 0 , the roots are found by x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} .
  • Step 2: For our specific function, identify: a=2 a = 2 , b=4 b = -4 , c=2 c = 2 .

Let's calculate the discriminant: b24ac=(4)24×2×2=1616=0 b^2 - 4ac = (-4)^2 - 4 \times 2 \times 2 = 16 - 16 = 0 .
Since the discriminant is zero, we have a repeated root, which means the graph touches the x-axis at one point.

The root is found as follows:
x=(4)±02×2=44=1 x = \frac{-(-4) \pm \sqrt{0}}{2 \times 2} = \frac{4}{4} = 1 .

This root x=1 x = 1 represents a vertex-touching parabola, with no intervals of x x such that y<0 y < 0 .

To find the positive domain (y>0 y > 0 ), we note the parabola y=2x24x+2 y = 2x^2 - 4x + 2 opens upwards (since a=2>0 a = 2 > 0 ) and only touches the x-axis at one point (1,0) (1, 0) . Thus, the positive domain is x1 x \ne 1 .

The function does not take any negative values, since it opens upwards and only touches the x-axis.

Therefore, the positive domain is x>0:x1 x > 0 :x \ne 1 and the negative domain is x<0: x < 0 : none.

In conclusion, the solution to the problem is:

x>0:x1 x > 0 :x \ne 1

x<0: x < 0 : none

3

Final Answer

x>0:x1 x > 0 :x\ne1

x<0: x < 0 : none

Key Points to Remember

Essential concepts to master this topic
  • Discriminant Rule: When b24ac=0 b^2 - 4ac = 0 , parabola touches x-axis once
  • Vertex Formula: Root at x=b2a=44=1 x = \frac{-b}{2a} = \frac{4}{4} = 1
  • Sign Check: Test values: y(0)=2>0 y(0) = 2 > 0 , y(2)=2>0 y(2) = 2 > 0

Common Mistakes

Avoid these frequent errors
  • Confusing positive/negative domains with x-values
    Don't think positive domain means x > 0 = wrong interpretation! The question asks where y > 0 (function values), not x-values. Always identify where the function output is positive or negative, not the input values.

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

What does 'positive domain' actually mean?

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The positive domain means all x-values where y > 0 (the function output is positive). It's about the function's values, not whether x is positive!

Why is the answer 'x ≠ 1' and not 'x > 1'?

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Since the parabola opens upward and only touches the x-axis at x = 1, the function is positive everywhere except at x = 1 where y = 0. So y > 0 for all x except x = 1.

How do I know the parabola opens upward?

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Look at the coefficient of x2 x^2 ! Since a = 2 > 0, the parabola opens upward. If a < 0, it would open downward.

What if the discriminant was positive instead of zero?

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With a positive discriminant, you'd get two distinct roots. The parabola would cross the x-axis twice, creating intervals where y > 0 and y < 0.

Why is there no negative domain?

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Since this upward-opening parabola only touches the x-axis at one point, it never goes below the x-axis. Therefore, y is never negative - only positive or zero.

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