Even and Odd Powers.

Calculus Level 3

Let n n be a positive integer.

(1): Let f ( x ) = x 2 n + 1 \large f(x) = x^{2n + 1} and g ( x ) = { log b ( x ) i f x > 0 log b ( x ) i f x < 0 g(x) = \large \begin{cases} \log_{b}(x) \:\ if \:\ x > 0 \\ -\log_{b}(-x) \:\ if \:\ x < 0 \end{cases}

f ( x ) f(x) and g ( x ) g(x) have common tangents at points A A and A A' .

Let A n A_{n} be the area of the region bounded by f ( x ) f(x) and the line A A AA' and L = lim n A n L = \lim_{n \rightarrow \infty} A_{n} .

(2): Let f ( x ) = x 2 n f(x) = x^{2n} and g ( x ) = log b x g(x) = \log_{b}|x| .

f ( x ) f(x) and g ( x ) g(x) have common tangents at points B B and B B' .

Let A n A^{*}_{n} be the area of the region bounded by f ( x ) f(x) and the line B B BB' and L = lim n A n L^{*} = \lim_{n \rightarrow \infty} A^{*}_{n} .

Find: L L \dfrac{L^{*}}{L} . .


The answer is 2.

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1 solution

Rocco Dalto
Dec 15, 2018

Let f ( x ) = x 2 n + 1 f(x) = x^{2n + 1} and g ( x ) = log b ( x ) = ln ( x ) ln ( b ) g(x) = \log_{b}(x) = \dfrac{\ln(x)}{\ln(b)} for x > 0 x > 0

f ( a ) = ( 2 n + 1 ) a 2 n = g ( a ) = 1 a ln ( b ) \implies f'(a) = (2n + 1)a^{2n} = g'(a) = \dfrac{1}{a\ln(b)} \implies a 2 n + 1 = 1 ( 2 n + 1 ) ln ( b ) a = ( 1 ( 2 n + 1 ) ln ( b ) ) 1 2 n + 1 a^{2n + 1} = \dfrac{1}{(2n + 1)\ln(b)} \implies a = (\dfrac{1}{(2n + 1)\ln(b)})^{\frac{1}{2n + 1}}

and

a 2 n + 1 = ln ( a ) ln ( b ) 1 ( 2 n + 1 ) ln ( b ) = ln ( 1 ( 2 n + 1 ) ln ( b ) ) ( 2 n + 1 ) ln ( b ) ln ( 1 ( 2 n + 1 ) ln ( b ) ) = 1 ln ( b ) = 1 2 n + 1 ) e a^{2n + 1} = \dfrac{\ln(a)}{\ln(b)} \implies \dfrac{1}{(2n + 1)\ln(b)} = \dfrac{\ln(\dfrac{1}{(2n + 1)\ln(b)})}{(2n + 1)\ln(b)} \implies \ln(\dfrac{1}{(2n + 1)\ln(b)}) = 1 \implies \ln(b) = \dfrac{1}{2n + 1)e} \implies

b = e 1 ( 2 n + 1 ) e a = e 1 2 n + 1 a 2 n + 1 = e \large b = e^{\frac{1}{(2n + 1)e}} \implies a = e^{\frac{1}{2n + 1}} \implies a^{2n + 1} = e

A : ( e 1 2 n + 1 , e ) \implies \large A:(e^{\frac{1}{2n + 1}}, e) and using the symmetry about the origin we have A : ( e 1 2 n + 1 , e ) \large A':(-e^{\frac{1}{2n + 1}}, -e)

m A A = e 2 n 2 n + 1 y = e 2 n 2 n + 1 x \implies \large m_{AA'} = e^{\frac{2n}{2n + 1}} \implies y = e^{\frac{2n}{2n + 1}}x

A n = 2 0 e 1 2 n + 1 ( e 2 n 2 n + 1 x x 2 n + 1 ) d x = ( n n + 1 ) e 2 n + 2 2 n + 1 \large \implies A_{n} = 2\displaystyle\int_{0}^{e^{\frac{1}{2n + 1}}} (e^{\frac{2n}{2n + 1}}x - x^{2n + 1}) \:\ dx = (\dfrac{n}{n + 1})e^{\frac{2n + 2}{2n + 1}}

L = lim n A n = e lim n ( 1 1 n + 1 ) e 1 2 n + 1 = e \implies \large L = \lim_{n \rightarrow \infty} A_{n} = e\lim_{n \rightarrow \infty} (1 - \dfrac{1}{n + 1})e^{\frac{1}{2n + 1}} = e .

Using the symmetry about the y y axis we have:

f ( x ) = x 2 n f(x) = x^{2n} and g ( x ) = log b ( x ) = ln ( x ) ln ( b ) f ( a ) = 2 n a 2 n 1 = g ( a ) = 1 a ln ( b ) a 2 n = 1 2 n ln ( b ) a = ( 1 2 n ln ( b ) ) 1 2 n g(x) = \log_{b}(x) = \dfrac{\ln(x)}{\ln(b)} \implies f'(a) = 2na^{2n - 1} = g'(a) = \dfrac{1}{a\ln(b)} \implies a^{2n} = \dfrac{1}{2n\ln(b)} \implies a = (\dfrac{1}{2n\ln(b)})^{\frac{1}{2n}}

and

a 2 n = ln ( a ) ln ( b ) 1 2 n ln ( b ) = ln ( 1 2 n ln ( b ) ) 2 n ln ( b ) a^{2n} = \dfrac{\ln(a)}{\ln(b)} \implies \dfrac{1}{2n\ln(b)} = \dfrac{\ln(\dfrac{1}{2n\ln(b)})}{2n\ln(b)}

1 = ln ( 1 2 n ln ( b ) ) \implies 1 = \ln(\dfrac{1}{2n\ln(b)}) \implies 2 n ln ( b ) = 1 e ln ( b ) = 1 2 n e b = e 1 2 n e a = e 1 2 n 2n\ln(b) = \dfrac{1}{e} \implies \ln(b) = \dfrac{1}{2ne} \implies b = e^{\frac{1}{2ne}} \implies a = e^{\frac{1}{2n}} and a 2 n = e a^{2n} = e .

B ( e 1 2 n , e ) \implies \large B(e^{\frac{1}{2n}}, e) and using the symmetry about the y y axis we have

B ( e 1 2 n , e ) \large B'(-e^{\frac{1}{2n}}, e) and line B B \large BB' is y = e \large y = e \implies A n = 2 0 e 1 2 n ( e x 2 n ) d x = 2 ( 2 n 2 n + 1 ) e 2 n + 1 2 n \large A^{*}_{n} = \displaystyle 2\int_{0}^{e^{\frac{1}{2n}}} (e - x^{2n}) \:\ dx = 2(\dfrac{2n}{2n + 1})e^{\frac{2n + 1}{2n}}

L = lim n A n = 2 e lim n ( 1 1 2 n + 1 ) e 1 2 n = 2 e \implies \large L^{*} = \lim_{n \rightarrow \infty} A^{*}_{n} = 2e\lim_{n \rightarrow \infty} (1 - \dfrac{1}{2n + 1})e^{\frac{1}{2n}} = 2e

L L = 2 \therefore \dfrac{L^{*}}{L} = \boxed{2} .

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