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What can be added to \( f(x)=2 x^{2}-3 x \) so that \( (2 x-7) \) and \( (x+5) \) are factors of \( f(x) \) ?

Ask by Norris Wheeler. in South Africa
Mar 11,2025

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Tutor-Verified Answer

Answer

The value to add is -35.

Solution

To determine what can be added to \( f(x) = 2x^2 - 3x \) so that \( (2x - 7) \) and \( (x + 5) \) are factors of the resulting polynomial, we can follow these steps: 1. **Identify the desired polynomial**: If \( (2x - 7) \) and \( (x + 5) \) are factors, then the polynomial can be expressed as: \[ f(x) + k = (2x - 7)(x + 5) \] where \( k \) is the value we need to find. 2. **Expand the right-hand side**: We will first expand \( (2x - 7)(x + 5) \): \[ (2x - 7)(x + 5) = 2x^2 + 10x - 7x - 35 = 2x^2 + 3x - 35 \] 3. **Set up the equation**: Now we can set up the equation: \[ f(x) + k = 2x^2 + 3x - 35 \] Substituting \( f(x) = 2x^2 - 3x \): \[ 2x^2 - 3x + k = 2x^2 + 3x - 35 \] 4. **Solve for \( k \)**: To find \( k \), we can equate the coefficients: \[ -3x + k = 3x - 35 \] Rearranging gives: \[ k = 3x - 35 + 3x = 6x - 35 \] 5. **Find the constant value of \( k \)**: Since \( k \) must be a constant, we can evaluate it at any value of \( x \). For simplicity, let's evaluate it at \( x = 0 \): \[ k = 6(0) - 35 = -35 \] Thus, the value that can be added to \( f(x) = 2x^2 - 3x \) so that \( (2x - 7) \) and \( (x + 5) \) are factors is \( -35 \). Therefore, the final answer is: \[ \boxed{-35} \]

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Beyond the Answer

To ensure that both \( (2x - 7) \) and \( (x + 5) \) are factors of \( f(x) = 2x^2 - 3x + k \), we can use the property that if \( r \) is a root of the polynomial, then the polynomial evaluated at \( r \) is zero. First, we find the roots of each factor: 1. For \( 2x - 7 = 0 \) ⇒ \( x = \frac{7}{2} \) 2. For \( x + 5 = 0 \) ⇒ \( x = -5 \) Now, we substitute these values into \( f(x) \): 1. Substituting \( x = \frac{7}{2} \): \[ f\left(\frac{7}{2}\right) = 2\left(\frac{7}{2}\right)^2 - 3\left(\frac{7}{2}\right) + k = 0 \] Simplifying this: \[ f\left(\frac{7}{2}\right) = 2 \cdot \frac{49}{4} - \frac{21}{2} + k = 0 \] \[ = \frac{49}{2} - \frac{21}{2} + k = 0 \] \[ \frac{28}{2} + k = 0 \] \[ k = -14 \] 2. Substituting \( x = -5 \): \[ f(-5) = 2(-5)^2 - 3(-5) + k = 0 \] Simplifying this: \[ f(-5) = 2 \cdot 25 + 15 + k = 0 \] \[ 50 + 15 + k = 0 \] \[ k = -65 \] To satisfy both conditions, we need a different approach since \( k = -14 \) and \( k = -65 \) are contradictory. Therefore, it is impossible for both \( (2x - 7) \) and \( (x + 5) \) to be factors of the polynomial \( f(x) = 2x^2 - 3x \) without changing its form. Thus, you need to modify \( f(x) \) accordingly, or select only one of the factors to proceed.

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