Everything But the Kitchen Sink

Calculus Level 5

ln [ lim x 0 ( 1 x 0 x ( e 2 t cos ( 3 t ) + e 3 t sinh ( 2 t ) ) 1 / t d t ) ] \displaystyle \ln\left[ \lim_{x \rightarrow 0} \left( \dfrac{1}{x} \int_{0}^{x} \left( e^{2t}\cos(3t) + e^{3t}\sinh(2t) \right)^{1/t} \ \mathrm d t \right) \right]

Evaluate the above expression.

Image Credit: Wikimedia Kitchen Sink by Tomwsulcer


The answer is 4.

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1 solution

First look at the limit. Letting the integral be F ( x ) F(x) and f ( x ) = x f(x) = x , we have both of these functions going to 0 0 as x 0 x \rightarrow 0 , and thus we have a limit of the indeterminate form 0 0 . \frac{0}{0}. As both functions satisfy the conditions required to apply L'Hopital's rule, we apply this rule (and use the Fundamental Theorem of Calculus) to find that our limit is equal to

S = lim x 0 ( e 2 x cos ( 3 x ) + e 3 x sinh ( 2 x ) ) 1 x , S = \lim_{x \rightarrow 0} (e^{2x}\cos(3x) + e^{3x}\sinh(2x))^{\frac{1}{x}},

which is now of the indeterminate form 1 . 1^{\infty}. So now we need to look at what happens to ln ( S ) \ln(S) in the limit as x 0. x \rightarrow 0. We then end up with

lim x 0 ln ( S ) = lim x 0 ln ( e 2 x cos ( 3 x ) + e 3 x sinh ( 2 x ) ) x \lim_{x \rightarrow 0} \ln(S) = \lim_{x \rightarrow 0} \dfrac{\ln(e^{2x}\cos(3x) + e^{3x}\sinh(2x))}{x} ,

which is of the indeterminate form 0 0 \frac{0}{0} , so apply L'Hopital's to get

lim x 0 2 e 2 x cos ( 3 x ) 3 e 2 x sin ( 3 x ) + 3 e 3 x sinh ( 2 x ) + 2 e 3 x cosh ( 2 x ) e 2 x cos ( 3 x ) + e 3 x sinh ( 2 x ) = 4. \lim_{x \rightarrow 0} \dfrac{2e^{2x}\cos(3x) - 3e^{2x}\sin(3x) + 3e^{3x}\sinh(2x) + 2e^{3x}\cosh(2x)}{e^{2x}\cos(3x) + e^{3x}\sinh(2x)} = 4.

But since we must take the natural log anyway we just have 4 \boxed{4} as our final answer.

The title of the question cracked me up :D ...I think it's supposed to convey that the question has nearly everything thrown in...Right?

Shashwat Shukla - 6 years, 2 months ago

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Yes, exactly. :)

Brian Charlesworth - 6 years, 2 months ago

Nice solution! Having ln \ln outside of the limit confused me a bit, but then it struck me.

Jake Lai - 6 years, 2 months ago

In the mean time I was wondering the application of kitchen :p

Krishna Sharma - 6 years, 2 months ago

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If there were a "kitchen sink" function I'd have thrown that into the problem as well. :)

Brian Charlesworth - 6 years, 2 months ago

Why the kitchen sink when you can just go to the hospital?

Jake Lai - 6 years, 2 months ago

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