Determine X for y=x²-4x+5 When f(x) < 0

Quadratic Functions with Negative Value Analysis

Look at the function below:

y=x24x+5 y=x^2-4x+5

Determine for which values of x x the following is true:

f(x)<0 f(x) < 0

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Step-by-step written solution

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1

Understand the problem

Look at the function below:

y=x24x+5 y=x^2-4x+5

Determine for which values of x x the following is true:

f(x)<0 f(x) < 0

2

Step-by-step solution

To solve this problem, we first consider the function y=x24x+5 y = x^2 - 4x + 5 . This is a quadratic function, and we can analyze the parabola it represents.

The standard form of a quadratic function is y=ax2+bx+c y = ax^2 + bx + c , where in our case a=1 a = 1 , b=4 b = -4 , and c=5 c = 5 .

To determine if the function has any negative values, we first find the vertex of the parabola. The vertex form of a quadratic function is determined by:

  • The x x -coordinate of the vertex is given by x=b2a x = -\frac{b}{2a} .

Plugging in our values:

x=42×1=42=2 x = -\frac{-4}{2 \times 1} = \frac{4}{2} = 2 .

The y y -coordinate of the vertex can be found by substituting x=2 x = 2 back into the equation:

y=(2)24(2)+5=48+5=1 y = (2)^2 - 4(2) + 5 = 4 - 8 + 5 = 1 .

The vertex of the parabola is (2,1) (2, 1) , which means the minimum point of the parabola is above the x-axis at y=1 y = 1 .

Next, we assess whether there are any real roots by finding the discriminant Δ \Delta :

Δ=b24ac=(4)24×1×5=1620=4 \Delta = b^2 - 4ac = (-4)^2 - 4 \times 1 \times 5 = 16 - 20 = -4 .

Since the discriminant is negative (Δ<0 \Delta < 0 ), this indicates the parabola does not intersect the x-axis at any real point. Therefore, it never dips below the x-axis.

Given that the vertex is above the x-axis and the discriminant is negative, the quadratic function y=x24x+5 y = x^2 - 4x + 5 is never negative for any real x x .

The function has no negative values.

3

Final Answer

The function has no negative values.

Key Points to Remember

Essential concepts to master this topic
  • Vertex Rule: Find minimum using x=b2a x = -\frac{b}{2a} for upward parabolas
  • Discriminant Test: Calculate Δ=b24ac=1620=4 \Delta = b^2 - 4ac = 16 - 20 = -4 to check x-intercepts
  • Verify Minimum: Substitute vertex x-value: y=(2)24(2)+5=1>0 y = (2)^2 - 4(2) + 5 = 1 > 0

Common Mistakes

Avoid these frequent errors
  • Assuming all quadratics have negative values
    Don't assume every parabola crosses the x-axis = wrong conclusion about negative values! When the discriminant is negative and the parabola opens upward, the function stays entirely above the x-axis. Always check the vertex position and discriminant sign together.

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

How can I tell if a quadratic has negative values without graphing?

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Use the discriminant test! If Δ=b24ac<0 \Delta = b^2 - 4ac < 0 and a>0 a > 0 , the parabola opens upward and never touches the x-axis, so it has no negative values.

What does it mean when the discriminant is negative?

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A negative discriminant means the quadratic has no real roots - the parabola doesn't cross the x-axis at any point. This is key information for determining if the function can be negative!

Why do I need to find the vertex?

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The vertex shows the minimum point of an upward-opening parabola. If this lowest point is above the x-axis (y>0 y > 0 ), then the entire function stays positive.

Can a quadratic function be always negative?

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Yes! If a<0 a < 0 (parabola opens downward) and the discriminant is negative, the function is always negative. But in our case, a=1>0 a = 1 > 0 , so it opens upward.

What if I got confused about which direction the parabola opens?

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Look at the coefficient of x2 x^2 ! If it's positive (like our +1), the parabola opens upward (U-shape). If negative, it opens downward (∩-shape).

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