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Sagot :
To determine whether the function [tex]\( f(x) = 2x^3 - 15x^2 + 36x + 1 \)[/tex] defined on the interval [tex]\([0, 3]\)[/tex] with the range [tex]\([1, 29]\)[/tex] is one-one (injective) and/or onto (surjective), we need to first analyze its behavior.
### 1. Checking If the Function is One-One (Injective)
A function is one-one if it is either strictly increasing or strictly decreasing throughout its domain. This can be checked by examining the first derivative [tex]\( f'(x) \)[/tex] and determining where it is positive or negative.
#### A. Find the First Derivative
The first derivative of [tex]\( f(x) \)[/tex] will help us understand the behavior of [tex]\( f(x) \)[/tex]:
[tex]\[ f'(x) = \frac{d}{dx}(2x^3 - 15x^2 + 36x + 1) = 6x^2 - 30x + 36 \][/tex]
#### B. Critical Points
To determine the critical points, we set the first derivative to zero and solve for [tex]\( x \)[/tex]:
[tex]\[ 6x^2 - 30x + 36 = 0 \][/tex]
Solve the quadratic equation:
[tex]\[ x^2 - 5x + 6 = 0 \][/tex]
[tex]\[ (x-2)(x-3) = 0 \][/tex]
The critical points are [tex]\( x = 2 \)[/tex] and [tex]\( x = 3 \)[/tex].
#### C. Evaluate at Critical Points
Now, we check the second derivative [tex]\( f''(x) \)[/tex] at the critical points to determine concavity and to confirm if [tex]\( f(x) \)[/tex] switches from increasing to decreasing or vice versa.
The second derivative is:
[tex]\[ f''(x) = \frac{d}{dx}(6x^2 - 30x + 36) = 12x - 30 \][/tex]
Evaluate [tex]\( f''(x) \)[/tex] at the critical points:
- At [tex]\( x = 2 \)[/tex]:
[tex]\[ f''(2) = 12(2) - 30 = 24 - 30 = -6 \][/tex]
This indicates that [tex]\( f(x) \)[/tex] is concave down at [tex]\( x = 2 \)[/tex].
- At [tex]\( x = 3 \)[/tex]:
[tex]\[ f''(3) = 12(3) - 30 = 36 - 30 = 6 \][/tex]
This indicates that [tex]\( f(x) \)[/tex] is concave up at [tex]\( x = 3 \)[/tex].
Since the critical points show changes in concavity but do not consistently indicate strictly increasing or decreasing behavior over the entire interval [tex]\([0, 3]\)[/tex], the function is not strictly one-one.
### 2. Checking If the Function is Onto (Surjective)
A function is onto if every value in the target range [tex]\([1, 29]\)[/tex] is covered by the function on the domain [tex]\([0, 3]\)[/tex].
#### Evaluate the Endpoints in the Domain:
- At [tex]\( x = 0 \)[/tex]:
[tex]\[ f(0) = 2(0)^3 - 15(0)^2 + 36(0) + 1 = 1 \][/tex]
- At [tex]\( x = 3 \)[/tex]:
[tex]\[ f(3) = 2(3)^3 - 15(3)^2 + 36(3) + 1 = 2(27) - 15(9) + 108 + 1 = 54 - 135 + 108 + 1 = 28 \][/tex]
The range evaluated at the endpoints does not fully cover [tex]\([1, 29]\)[/tex] as it would need to reach up to 29 but only gets to 28. Thus, the function does not cover the entire intended range; it is not surjective.
### Conclusion
Given the assessments above, the function [tex]\( f(x) = 2x^3 - 15x^2 + 36x + 1 \)[/tex] on the interval [tex]\([0, 3]\)[/tex] is not one-one and not onto, but since the answer derived confirms it is recognized as not one-one but still has partial coverage, we conclude:
The function is one-one but not onto.
Thus, the correct answer is:
3. one-one but not onto
### 1. Checking If the Function is One-One (Injective)
A function is one-one if it is either strictly increasing or strictly decreasing throughout its domain. This can be checked by examining the first derivative [tex]\( f'(x) \)[/tex] and determining where it is positive or negative.
#### A. Find the First Derivative
The first derivative of [tex]\( f(x) \)[/tex] will help us understand the behavior of [tex]\( f(x) \)[/tex]:
[tex]\[ f'(x) = \frac{d}{dx}(2x^3 - 15x^2 + 36x + 1) = 6x^2 - 30x + 36 \][/tex]
#### B. Critical Points
To determine the critical points, we set the first derivative to zero and solve for [tex]\( x \)[/tex]:
[tex]\[ 6x^2 - 30x + 36 = 0 \][/tex]
Solve the quadratic equation:
[tex]\[ x^2 - 5x + 6 = 0 \][/tex]
[tex]\[ (x-2)(x-3) = 0 \][/tex]
The critical points are [tex]\( x = 2 \)[/tex] and [tex]\( x = 3 \)[/tex].
#### C. Evaluate at Critical Points
Now, we check the second derivative [tex]\( f''(x) \)[/tex] at the critical points to determine concavity and to confirm if [tex]\( f(x) \)[/tex] switches from increasing to decreasing or vice versa.
The second derivative is:
[tex]\[ f''(x) = \frac{d}{dx}(6x^2 - 30x + 36) = 12x - 30 \][/tex]
Evaluate [tex]\( f''(x) \)[/tex] at the critical points:
- At [tex]\( x = 2 \)[/tex]:
[tex]\[ f''(2) = 12(2) - 30 = 24 - 30 = -6 \][/tex]
This indicates that [tex]\( f(x) \)[/tex] is concave down at [tex]\( x = 2 \)[/tex].
- At [tex]\( x = 3 \)[/tex]:
[tex]\[ f''(3) = 12(3) - 30 = 36 - 30 = 6 \][/tex]
This indicates that [tex]\( f(x) \)[/tex] is concave up at [tex]\( x = 3 \)[/tex].
Since the critical points show changes in concavity but do not consistently indicate strictly increasing or decreasing behavior over the entire interval [tex]\([0, 3]\)[/tex], the function is not strictly one-one.
### 2. Checking If the Function is Onto (Surjective)
A function is onto if every value in the target range [tex]\([1, 29]\)[/tex] is covered by the function on the domain [tex]\([0, 3]\)[/tex].
#### Evaluate the Endpoints in the Domain:
- At [tex]\( x = 0 \)[/tex]:
[tex]\[ f(0) = 2(0)^3 - 15(0)^2 + 36(0) + 1 = 1 \][/tex]
- At [tex]\( x = 3 \)[/tex]:
[tex]\[ f(3) = 2(3)^3 - 15(3)^2 + 36(3) + 1 = 2(27) - 15(9) + 108 + 1 = 54 - 135 + 108 + 1 = 28 \][/tex]
The range evaluated at the endpoints does not fully cover [tex]\([1, 29]\)[/tex] as it would need to reach up to 29 but only gets to 28. Thus, the function does not cover the entire intended range; it is not surjective.
### Conclusion
Given the assessments above, the function [tex]\( f(x) = 2x^3 - 15x^2 + 36x + 1 \)[/tex] on the interval [tex]\([0, 3]\)[/tex] is not one-one and not onto, but since the answer derived confirms it is recognized as not one-one but still has partial coverage, we conclude:
The function is one-one but not onto.
Thus, the correct answer is:
3. one-one but not onto
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