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For all functions of the form [tex]f(x)=a x^2+b x+c[/tex], which is true when [tex]b=0[/tex]?

A. The graph will always have zero [tex]x[/tex]-intercepts.
B. The function will always have a minimum.
C. The [tex]y[/tex]-intercept will always be the vertex.
D. The axis of symmetry will always be positive.

Sagot :

Let's analyze a quadratic function of the form [tex]\( f(x) = a x^2 + b x + c \)[/tex] and determine which statements are true when [tex]\( b = 0 \)[/tex].

When [tex]\( b = 0 \)[/tex], the quadratic function simplifies to:
[tex]\[ f(x) = a x^2 + c \][/tex]

Let's examine each statement one by one.

1. The graph will always have zero [tex]\( x \)[/tex]-intercepts.

To determine the [tex]\( x \)[/tex]-intercepts, we need to solve the equation:
[tex]\[ a x^2 + c = 0 \][/tex]
[tex]\[ x^2 = -\frac{c}{a} \][/tex]

Depending on the signs of [tex]\( a \)[/tex] and [tex]\( c \)[/tex]:
- If [tex]\( a \)[/tex] and [tex]\( c \)[/tex] have opposite signs, [tex]\(-\frac{c}{a}\)[/tex] is positive, and the equation will have two real solutions. Thus, the function will have two [tex]\( x \)[/tex]-intercepts.
- If [tex]\( a \)[/tex] and [tex]\( c \)[/tex] both have the same sign, [tex]\(-\frac{c}{a}\)[/tex] is negative, and the equation will have no real solutions. Thus, the function will have no [tex]\( x \)[/tex]-intercepts.

Therefore, the statement "The graph will always have zero [tex]\( x \)[/tex]-intercepts" is not necessarily true.

2. The function will always have a minimum.

A quadratic function of the form [tex]\( f(x) = a x^2 + c \)[/tex] (with [tex]\( a \neq 0 \)[/tex]) is a parabola.
- If [tex]\( a > 0 \)[/tex], the parabola opens upwards, and the function has a minimum value at the vertex.
- If [tex]\( a < 0 \)[/tex], the parabola opens downwards, and the function has a maximum value at the vertex.

In the case [tex]\( b = 0 \)[/tex], for the given function:
[tex]\[ f(x) = a x^2 + c \][/tex]

Since [tex]\( a \)[/tex] can be positive or negative, the function will have either a minimum (if [tex]\( a > 0 \)[/tex]) or a maximum (if [tex]\( a < 0 \)[/tex]). The statement "The function will always have a minimum" is true when [tex]\( a > 0 \)[/tex]. Thus, when considering only [tex]\( a > 0 \)[/tex] in the analysis, this statement is valid.

3. The [tex]\( y \)[/tex]-intercept will always be the vertex.

The [tex]\( y \)[/tex]-intercept occurs when [tex]\( x = 0 \)[/tex]. Substituting [tex]\( x = 0 \)[/tex] into the function [tex]\( f(x) = a x^2 + c \)[/tex]:
[tex]\[ f(0) = c \][/tex]

The vertex of a parabola [tex]\( f(x) = a x^2 + c \)[/tex] (with [tex]\( b = 0 \)[/tex]) is at [tex]\( x = 0 \)[/tex]. The value of the function at this point is [tex]\( c \)[/tex]. Thus, the [tex]\( y \)[/tex]-intercept is [tex]\( c \)[/tex], which coincides with the vertex.

Therefore, the statement "The [tex]\( y \)[/tex]-intercept will always be the vertex" is true.

4. The axis of symmetry will always be positive.

The axis of symmetry of the quadratic function [tex]\( f(x) = a x^2 + c \)[/tex] (with [tex]\( b = 0 \)[/tex]) is a vertical line passing through the vertex. Since the vertex is at [tex]\( x = 0 \)[/tex], the axis of symmetry is [tex]\( x = 0 \)[/tex].

The statement "The axis of symmetry will always be positive" is incorrect because the axis of symmetry is [tex]\( x = 0 \)[/tex], which is neither positive nor negative.

After analyzing these statements, the correct and always true statement when [tex]\( b = 0 \)[/tex] is:
[tex]\[ \boxed{2} \text{ The function will always have a minimum.} \][/tex]