Question
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2. Let \( f: R, \rightarrow[-5, x) \) be defined as \( f(x)=9 x^{2}+6 x-5 \), where \( R_{+} \), is the set of all non-negative \( r \) n numbers. Then, \( f \) is: \( \begin{array}{ll}\text { (d) one-one } & \text { (b) onto } \\ \text { (d) bijective } & \text { (d) neither one-one nor onto }\end{array} \)

Ask by Bush Carroll. in India
Jan 24,2025

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Tutor-Verified Answer

Answer

\( f \) is bijective.

Solution

Let's analyze the function \( f: \mathbb{R}_+ \rightarrow [-5, \infty) \) defined by \( f(x) = 9x^2 + 6x - 5 \), where \( \mathbb{R}_+ \) is the set of all non-negative real numbers. ### 1. **One-One (Injective)** A function is **one-one** if each element of the domain maps to a unique element in the codomain. - **Derivative Analysis:** \( f'(x) = 18x + 6 \) For \( x \geq 0 \), \( f'(x) \geq 6 \), which means the function is **strictly increasing** on \( \mathbb{R}_+ \). - **Conclusion:** Since the function is strictly increasing, it is **one-one**. ### 2. **Onto (Surjective)** A function is **onto** if every element in the codomain has a pre-image in the domain. - **Range of \( f(x) \):** \( f(0) = 9(0)^2 + 6(0) - 5 = -5 \) As \( x \) increases, \( f(x) \) increases without bound, approaching \( \infty \). - **Conclusion:** The range of \( f(x) \) is \( [-5, \infty) \), which exactly matches the codomain. Therefore, \( f \) is **onto**. ### 3. **Bijective** A function is **bijective** if it is both one-one and onto. - **Conclusion:** Since \( f \) is both one-one and onto, it is **bijective**. **Final Answer:** \( f \) is **bijective**. **Answer:** \( f \) is **bijective**.

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Beyond the Answer

To determine the nature of the function \( f(x) = 9x^2 + 6x - 5 \) defined on \( R_{+} \), we first examine its behavior. Since it is a quadratic function with a positive leading coefficient, it opens upwards and has a minimum point, indicating that it is not one-one over its entire range. Additionally, we find that it’s not onto since its range is restricted to \( [-5, x) \), so it cannot cover all real numbers. Also, exploring its vertex, the minimum value occurs at \( x = -\frac{b}{2a} = -\frac{6}{2 \cdot 9} = -\frac{1}{3} \), which is outside the non-negative domain. Therefore, the range begins at its minimum value, confirming it does not meet the criteria for being onto either. Thus, \( f \) is neither one-one nor onto.

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