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 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

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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