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17. [-/2 Points] DETAILS \( \begin{array}{l}\text { MY NOTES } \\ \text { Find the values of } a \text { and } b \text { that make } f \text { continuous everywhere. } \\ \qquad f(x)=\left\{\begin{array}{ll}\frac{x^{2}-4}{x-2} & \text { if } x<2 \\ a x^{2}-b x+3 & \text { if } 2 \leq x<3 \\ 2 x-a+b & \text { if } x \geq 3\end{array}\right. \\ a=\square \\ b=\square\end{array} \)

Ask by Griffin Bradley. in the United States
Feb 25,2025

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Answer

a = ½ and b = ½

Solution

We need to find a and b so that the function   f(x) = { (x²–4)/(x–2) if x < 2        a·x² – b·x + 3  if 2 ≤ x < 3        2x – a + b   if x ≥ 3 is continuous for all x. This means we need to ensure continuity at the transition points x = 2 and x = 3. Step 1. Continuity at x = 2 For x < 2: f(x) = (x² – 4)/(x – 2). Notice that for x ≠ 2, the numerator factors as (x – 2)(x + 2), so   (x² – 4)/(x – 2) = x + 2. Thus, the left-hand limit as x → 2⁻ is:   limₓ→2⁻ f(x) = 2 + 2 = 4. For x ≥ 2 (the middle piece): f(x) = a·x² – b·x + 3. Evaluating at x = 2 gives:   f(2) = 4a – 2b + 3. For continuity at x = 2 we require:   4a – 2b + 3 = 4. Subtracting 3 from both sides:   4a – 2b = 1            (Equation 1) Step 2. Continuity at x = 3 For x in [2,3): the middle piece gives:   f(3) = a·(3)² – b·(3) + 3 = 9a – 3b + 3. For x ≥ 3 the function is given by:   f(x) = 2x – a + b. Evaluating at x = 3 gives:   f(3) = 6 – a + b. Setting these equal for continuity at x = 3:   9a – 3b + 3 = 6 – a + b. Bring like terms together:   9a + a – 3b – b = 6 – 3,   10a – 4b = 3         (Equation 2) Step 3. Solve the system of equations We have:   Equation 1: 4a – 2b = 1  or 2a – b = ½   Equation 2: 10a – 4b = 3 Solve Equation 1 for b:   b = 2a – ½. Substitute b into Equation 2:   10a – 4(2a – ½) = 3,   10a – 8a + 2 = 3,   2a + 2 = 3,   2a = 1,   a = ½. Now substitute a back into the expression for b:   b = 2(½) – ½ = 1 – ½ = ½. Answer:   a = ½ and b = ½ These values ensure that f(x) is continuous everywhere.

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The Deep Dive

To ensure that the function \( f(x) \) is continuous everywhere, we need to ensure that both the left-hand limit and the right-hand limit at the points of transition (specifically at \( x=2 \) and \( x=3 \)) match the function's value. 1. For \( x < 2 \), we can simplify \( f(x) \) as: \[ f(x) = \frac{x^2 - 4}{x - 2} = x + 2 \quad \text{(as long as \( x \neq 2 \))} \] Evaluating this as \( x \) approaches 2 gives: \[ \lim_{x \to 2^-} f(x) = 2 + 2 = 4 \] Moving to \( x = 2 \) for the second piece, we set \( f(2) \) equal to the left limit: \[ a(2)^2 - b(2) + 3 = 4 \] Simplifying, we get: \[ 4a - 2b + 3 = 4 \\ \Rightarrow 4a - 2b = 1 \quad \text{(Equation 1)} \] 2. Next, we consider the transition at \( x = 3 \): For the second piece, evaluating at 3 gives: \[ a(3)^2 - b(3) + 3 = 9a - 3b + 3 \] The right-hand limit from the third piece \( f(x) = 2x - a + b \) at \( x=3 \) is: \[ 2(3) - a + b = 6 - a + b \] Setting these equal for continuity: \[ 9a - 3b + 3 = 6 - a + b \\ \Rightarrow 10a - 4b + 3 = 6 \\ \Rightarrow 10a - 4b = 3 \quad \text{(Equation 2)} \] Now we solve these two simultaneous equations: From Equation 1: \[ 2b = 4a - 1 \\ b = 2a - 0.5 \] Substituting for \( b \) in Equation 2: \[ 10a - 4(2a - 0.5) = 3 \\ 10a - 8a + 2 = 3 \\ 2a = 1 \\ a = \frac{1}{2} \] Putting \( a \) back into the expression for \( b \): \[ b = 2\left(\frac{1}{2}\right) - 0.5 = 1 - 0.5 = 0.5 \] So, the final values are: \[ a = \frac{1}{2}, \quad b = \frac{1}{2} \]

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