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\( 1<\quad \begin{array}{l}\text { Find the range of the quadratic function and the maximum or minimum value of the function. Identify the intervals over } \\ \text { which the function is increasing or decreasing. } \\ y=2 x^{2}+5 x+3\end{array} \)

Ask by Clarke Lambert. in the United States
Feb 25,2025

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The quadratic function \( y = 2x^{2} + 5x + 3 \) has a minimum value of \( -\frac{1}{8} \) at \( x = -\frac{5}{4} \). The range of the function is all real numbers greater than or equal to \( -\frac{1}{8} \). The function is decreasing on \( (-\infty, -\frac{5}{4}) \) and increasing on \( (-\frac{5}{4}, \infty) \).

Solución

We are given the quadratic function   y = 2x² + 5x + 3. Step 1. Find the Vertex (the minimum point for this upward-opening parabola). • For a quadratic function y = ax² + bx + c, the x-coordinate of the vertex is given by x = –b/(2a). Here, a = 2 and b = 5, so   x = –5 / (2 × 2) = –5/4. • To find the y-coordinate, substitute x = –5/4 into the function:   y = 2(–5/4)² + 5(–5/4) + 3. Calculate each term:   – (–5/4)²: (–5/4)² = 25/16,   – First term: 2 × (25/16) = 50/16 = 25/8,   – Second term: 5 × (–5/4) = –25/4,   – Third term: 3 (which can be written as 24/8 to match denominators). Combine the terms:   y = 25/8 – (25/4) + 24/8    (Note that 25/4 = 50/8),   y = (25 – 50 + 24) / 8,   y = (–1) / 8,   y = –1/8. Thus, the vertex is at (–5/4, –1/8), which is the minimum point. Step 2. Determine the Range Since the coefficient of x² is positive (2 > 0), the parabola opens upward and has a minimum point at y = –1/8. Therefore, the range of the function is:   Range: [–1/8, ∞). Step 3. Identify the Increasing and Decreasing Intervals • Find the derivative of y with respect to x:   y' = d/dx (2x² + 5x + 3) = 4x + 5. • Set the derivative equal to zero to locate the critical point:   4x + 5 = 0 ⟹ x = –5/4. Examine the sign of y' on either side of x = –5/4:  • For x < –5/4, choose a test point (e.g., x = –2):    y' = 4(–2) + 5 = –8 + 5 = –3  (results in a negative derivative)    This means y is decreasing on (–∞, –5/4).  • For x > –5/4, choose a test point (e.g., x = 0):    y' = 4(0) + 5 = 5       (results in a positive derivative)    This means y is increasing on (–5/4, ∞). Summary: • The function has a minimum value of –1/8 at x = –5/4. • The range of the function is [–1/8, ∞). • The function is decreasing on the interval (–∞, –5/4) and increasing on the interval (–5/4, ∞).

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Beyond the Answer

To find the range of the quadratic function \( y = 2x^2 + 5x + 3 \), we first determine whether it opens upwards or downwards. Since the coefficient of \( x^2 \) is positive (2), the parabola opens upwards, indicating that it has a minimum value. We can find this minimum using the vertex formula \( x = -\frac{b}{2a} \). For our function, \( a = 2 \) and \( b = 5 \), so \( x = -\frac{5}{2(2)} = -\frac{5}{4} \). Substituting this back into the function gives us the minimum value: \[ y = 2\left(-\frac{5}{4}\right)^2 + 5\left(-\frac{5}{4}\right) + 3 = 2\left(\frac{25}{16}\right) - \frac{25}{4} + 3 = \frac{25}{8} - \frac{50}{8} + \frac{24}{8} = -\frac{1}{8} \] Thus, the minimum value is \( -\frac{1}{8} \). Since the parabola opens upwards, the range of the function is \( \left[-\frac{1}{8}, \infty\right) \). To determine where the function is increasing or decreasing, we find the derivative \( y' = 4x + 5 \). Setting this to zero to find critical points gives \( 4x + 5 = 0 \Rightarrow x = -\frac{5}{4} \). The function decreases for \( x < -\frac{5}{4} \) and increases for \( x > -\frac{5}{4} \). Thus, the intervals are: - Decreasing: \( (-\infty, -\frac{5}{4}) \) - Increasing: \( (-\frac{5}{4}, \infty) \)

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