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The function [tex]f[/tex] is defined by [tex]f(x) = a \sqrt{x + b}[/tex], where [tex]a[/tex] and [tex]b[/tex] are constants. In the [tex]xy[/tex]-plane, the graph of [tex]y = f(x)[/tex] passes through the point [tex](7, 0)[/tex] and [tex]f(14) \ \textless \ 0[/tex]. Which of the following must be true?

A. [tex]a \ \textless \ 0[/tex]
B. [tex]b \ \textless \ -7[/tex]
C. [tex]a \cdot b \ \textgreater \ 0[/tex]
D. [tex]b \ \textgreater \ 7[/tex]
E. [tex]a = 0[/tex]


Sagot :

To solve the problem, let's carefully go through the conditions provided and determine the constants [tex]\(a\)[/tex] and [tex]\(b\)[/tex] for the function [tex]\(f(x) = a \sqrt{x + b}\)[/tex]. We are given two key pieces of information:

1. The function passes through the point [tex]\((7, 0)\)[/tex], which implies [tex]\(f(7) = 0\)[/tex].
2. The function value at [tex]\(x = 14\)[/tex] is negative, i.e., [tex]\(f(14) < 0\)[/tex].

Let's start with the first piece of information:

### Step 1: Determine [tex]\(b\)[/tex]
At the point [tex]\((7, 0)\)[/tex]:
[tex]\[ f(7) = 0 \][/tex]

Substitute [tex]\(x = 7\)[/tex] into the function:
[tex]\[ f(7) = a \sqrt{7 + b} = 0 \][/tex]

For this equation to hold, the expression inside the square root must equal zero (since [tex]\(a\)[/tex] cannot be zero):
[tex]\[ \sqrt{7 + b} = 0 \][/tex]

Squaring both sides, we get:
[tex]\[ 7 + b = 0 \][/tex]

Solving for [tex]\(b\)[/tex]:
[tex]\[ b = -7 \][/tex]

### Step 2: Determine [tex]\(a\)[/tex]
Next, we use the second condition: [tex]\(f(14) < 0\)[/tex].

Substitute [tex]\(x = 14\)[/tex] and [tex]\(b = -7\)[/tex] into the function:
[tex]\[ f(14) = a \sqrt{14 + (-7)} < 0 \][/tex]
[tex]\[ f(14) = a \sqrt{7} < 0 \][/tex]

Since [tex]\(\sqrt{7}\)[/tex] is a positive number, for the product [tex]\(a \sqrt{7}\)[/tex] to be negative, [tex]\(a\)[/tex] must be negative.

### Conclusion
Given these analyses, we determined the following constants:
- [tex]\(b = -7\)[/tex]
- [tex]\(a\)[/tex] must be a negative value.

Thus, the constants [tex]\(a\)[/tex] and [tex]\(b\)[/tex] that satisfy the given conditions are:
[tex]\[ a = -1 \][/tex]
[tex]\[ b = -7 \][/tex]

So, the required constants for the function [tex]\(f(x) = a \sqrt{x + b}\)[/tex] are:
[tex]\[ a = -1 \][/tex]
[tex]\[ b = -7 \][/tex]

These values ensure that the function passes through the point [tex]\((7, 0)\)[/tex] and satisfies [tex]\(f(14) < 0\)[/tex].