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if \( F(n)=(x-4)^{3} \sqrt{n+3} \) \( \lim _{h \rightarrow 0} \frac{F^{2}(5-h)-3 F(5-h)+2}{h(5-h)}=? \)

Ask by Byrd Carrillo. in Iran
Feb 03,2025

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The limit evaluates to \( \frac{\left(3 - 4\sqrt{2}(x - 4)^3\right)(x - 4)^3}{20\sqrt{2}} \).

Solución

To evaluate the limit \[ \lim_{h \to 0} \frac{F^2(5-h) - 3F(5-h) + 2}{h(5-h)}, \] where \( F(n) = (x - 4)^3 \sqrt{n + 3} \), we'll follow these steps: 1. **Evaluate \( F(5) \) and \( F'(5) \):** \[ F(5) = (x - 4)^3 \sqrt{5 + 3} = (x - 4)^3 \cdot 2\sqrt{2}. \] The derivative of \( F(n) \) with respect to \( n \) is: \[ F'(n) = (x - 4)^3 \cdot \frac{1}{2\sqrt{n + 3}}. \] Thus, \[ F'(5) = (x - 4)^3 \cdot \frac{1}{2\sqrt{8}} = \frac{(x - 4)^3}{4\sqrt{2}}. \] 2. **Expand \( F(5 - h) \) around \( h = 0 \) using a linear approximation:** \[ F(5 - h) \approx F(5) - h F'(5). \] 3. **Expand \( F^2(5 - h) - 3F(5 - h) + 2 \):** \[ F^2(5 - h) - 3F(5 - h) + 2 \approx (F(5) - h F'(5))^2 - 3(F(5) - h F'(5)) + 2. \] Expanding and simplifying, the constant terms cancel out (assuming \( F(5)^2 - 3F(5) + 2 = 0 \)), leaving: \[ \approx (-2F(5)F'(5) + 3F'(5))h. \] 4. **Substitute into the original limit expression:** \[ \lim_{h \to 0} \frac{-2F(5)F'(5) + 3F'(5)}{h(5 - h)} \cdot h = \frac{-2F(5) + 3}{5} \cdot F'(5). \] 5. **Substitute \( F(5) \) and \( F'(5) \) back into the expression:** \[ \frac{3 - 2F(5)}{5} \cdot F'(5) = \frac{3 - 4\sqrt{2}(x - 4)^3}{5} \cdot \frac{(x - 4)^3}{4\sqrt{2}}. \] Simplifying: \[ = \frac{(3 - 4\sqrt{2}(x - 4)^3)(x - 4)^3}{20\sqrt{2}}. \] **Final Answer:** \[ \boxed{\frac{\left[\,3 - 4\,\sqrt{2}\, (x - 4)^3\,\right]\, (x - 4)^3}{\,20\,\sqrt{2}\,}} \]

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Mind Expander

To tackle the limit, we first need to find \( F(5-h) \): \[ F(n) = (x-4)^3 \sqrt{n+3} \] Substituting \( n = 5 - h \): \[ F(5-h) = (x-4)^3 \sqrt{(5-h)+3} = (x-4)^3 \sqrt{8-h} \] Next, we calculate \( F^2(5-h) \): \[ F^2(5-h) = \left((x-4)^3 \sqrt{8-h}\right)^2 = (x-4)^6 (8-h) \] Now we substitute \( F(5-h) \) and \( F^2(5-h) \) into the limit expression: \[ \lim_{h \rightarrow 0} \frac{(x-4)^6(8-h) - 3(x-4)^3 \sqrt{8-h} + 2}{h(5-h)} \] As \( h \) approaches \( 0 \), \( \sqrt{8-h} \) approaches \( \sqrt{8} = 2\sqrt{2} \). Therefore, let's simplify further: \[ F(5) = (x-4)^3 \cdot 2\sqrt{2} \] Substituting \( \sqrt{8-h} \) in the limit: \[ F^2(5) = (x - 4)^6 \cdot 8 \] So we have: \[ F^2(5-h) \rightarrow (x - 4)^6 \cdot 8, \quad F(5-h) \rightarrow (x - 4)^3 \cdot 2\sqrt{2} \] Then the limit simplifies to: \[ \lim_{h \rightarrow 0} \frac{(x-4)^6 \cdot 8 - 3(x-4)^3 \cdot 2\sqrt{2} + 2}{h(5-h)} \] Now, when \( h \) is very small, \( 5-h = 5 \) which behaves regularly, thus preventing the denominator from reaching zero. Evaluating \( (x-4)^6 \cdot 8 - 6(x-4)^3\sqrt{2} + 2 \) separately to find out whether the numerator approaches \( 0 \): 1. If \( (x-4)^3 - \sqrt{2} \) has the same root for \( (x-4) \). 2. It may approach zero, indicating L'Hôpital's rule may apply if the factor \( h(5-h) \) is linear. Apply L'Hôpital's Rule if necessary: \[ \text{Evaluate the derivatives of numerator and denominator, then recalculate the limit.} \] After careful evaluation, the value simplifies to either approach a constant or identify zero. This leads us to a formal limit evaluation. Ultimately the exact answer may depend on specified conditions of x or any other numerical values involved, lending specifics about exact continuity or value.

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