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Evaluate the limit:
[tex]\[ \lim_{x \rightarrow 0} 5 x^{\sqrt{9x}} \][/tex]


Sagot :

To solve the limit [tex]\(\lim_{x \rightarrow 0} 5 x^{\sqrt{9x}}\)[/tex], we need to carefully analyze the behavior of the function [tex]\(5 x^{\sqrt{9x}}\)[/tex] as [tex]\(x\)[/tex] approaches 0.

1. Expression Analysis:

The function we are interested in is [tex]\(5 x^{\sqrt{9x}}\)[/tex]. This is an exponential function with a base [tex]\(x\)[/tex] and an exponent that involves [tex]\(x\)[/tex].

2. Exponent Simplification:

Let's focus on the exponent [tex]\(\sqrt{9x}\)[/tex]. Simplifying, we get,
[tex]\[ \sqrt{9x} = (\sqrt{9} \cdot \sqrt{x}) = 3\sqrt{x} \][/tex]
Thus, the expression can be rewritten as:
[tex]\[ 5 x^{3\sqrt{x}} \][/tex]

3. Taking the Natural Logarithm:

To handle the complex exponentiation, we can take the natural logarithm of the function to simplify the exponent. Set [tex]\(y = x^{3\sqrt{x}}\)[/tex]. Then,
[tex]\[ \ln y = 3\sqrt{x} \cdot \ln x \][/tex]

4. Limit of the Exponential Part:

We now focus on finding the limit of [tex]\(\ln y\)[/tex] as [tex]\(x\)[/tex] approaches 0. Consider the term [tex]\(3\sqrt{x} \cdot \ln x\)[/tex]:
[tex]\[ \lim_{x \to 0} 3\sqrt{x} \ln x \][/tex]

5. Substitution for Simplification:

Let's perform a substitution to simplify the analysis. Set [tex]\(u = \sqrt{x}\)[/tex]. Hence, when [tex]\(x\)[/tex] approaches 0, [tex]\(u\)[/tex] also approaches 0. The expression [tex]\(3\sqrt{x} \ln x\)[/tex] becomes:
[tex]\[ 3u \ln (u^2) = 3u (2 \ln u) = 6u \ln u \][/tex]

6. Evaluating the Limit:

We need to evaluate:
[tex]\[ \lim_{u \to 0} 6u \ln u \][/tex]
Notice that as [tex]\(u\)[/tex] approaches 0, [tex]\(\ln u\)[/tex] approaches [tex]\(-\infty\)[/tex], and [tex]\(u\)[/tex] approaches 0; thus, [tex]\(6u \ln u\)[/tex] approaches 0.

7. Applying the Limit:

Since [tex]\(\ln y = 6u \ln u\)[/tex], it follows that:
[tex]\[ \lim_{x \to 0} \ln x^{3\sqrt{x}} = \lim_{u \to 0} 6u \ln u = 0 \][/tex]

8. Exponentiation to Reverse the Logarithm:

If [tex]\(\ln L = 0\)[/tex], then [tex]\(L = e^0 = 1\)[/tex]. Consequently:
[tex]\[ \lim_{x \to 0} x^{3\sqrt{x}} = 1 \][/tex]

9. Include the Constant Factor:

Therefore, taking into account the original multiplier:
[tex]\[ \lim_{x \rightarrow 0} 5 x^{3 \sqrt{x}} = 5 \times 1 = 5 \][/tex]

The limit of the function as [tex]\(x\)[/tex] approaches 0 is:
[tex]\[ \boxed{5} \][/tex]
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