λ \displaystyle \lambda and \wp under the ceilings!

Calculus Level 5

λ = 0 1 d x 1 + x 3 \displaystyle \lambda = \int_{0}^{1} \frac{dx}{1+x^3}

= lim n ( r = 1 r = n n 2 + r 2 n 2 ) 1 n \displaystyle \wp = \lim_{n \to \infty} \Bigg(\prod_{r=1}^{r=n} \frac{n^2 + r^2}{n^2}\Bigg)^{\frac{1}{n}} Then what is the value of 3 λ + ln \displaystyle \lceil 3 \lambda + \ln \wp \rceil


The answer is 3.

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1 solution

Deepanshu Gupta
Nov 5, 2014

Edited

ρ = lim n ( r = 1 r = n n 2 + r 2 n 2 ) 1 n ln ρ = lim n 1 n r = 1 n ln ( 1 + ( r n ) 2 ) \rho =\lim _{ n\to \infty } (\prod _{ r=1 }^{ r=n } \frac { n^{ 2 }+r^{ 2 } }{ n^{ 2 } } )^{ \frac { 1 }{ n } }\\ \\ \ln { \rho } =\quad \lim _{ n\to \infty } \quad \cfrac { 1 }{ n } \sum _{ r=1 }^{ n }{ \ln { (1+{ (\cfrac { r }{ n } ) }^{ 2 }) } } .

Now Using Riemann Sums :

ρ = lim n ( r = 1 r = n n 2 + r 2 n 2 ) 1 n ln ρ = lim n 1 n r = 1 n ln ( 1 + ( r n ) 2 ) ln ρ = 0 1 ln ( 1 + x 2 ) d x = l n 2 2 + π 2 . . . . ( 1 ) λ = 0 1 d x 1 + x 3 λ = 0 1 x 2 + 1 x 2 1 + x 3 d x = 1 3 0 1 3 x 2 d x 1 + x 3 + 0 1 ( 1 + x ) ( 1 x ) ( 1 + x ) ( x 2 + 1 x ) d x = 1 3 [ l n ( 1 + x 3 ) ] 0 1 + 0 1 1 x ( x 2 + 1 x ) d x 3 λ = ln 2 + π 3 . . . . . . . . ( 2 ) 3 λ + ln ρ = ln 2 + π 3 + l n 2 2 + π 2 = 3 \quad \rho =\lim _{ n\to \infty } (\prod _{ r=1 }^{ r=n } \frac { n^{ 2 }+r^{ 2 } }{ n^{ 2 } } )^{ \frac { 1 }{ n } }\\ \\ \ln { \rho } =\quad \lim _{ n\to \infty } \quad \cfrac { 1 }{ n } \sum _{ r=1 }^{ n }{ \ln { (1+{ (\cfrac { r }{ n } ) }^{ 2 }) } } \\ \\ \ln { \rho } \quad =\quad \int _{ 0 }^{ 1 }{ \ln { (1+{ x }^{ 2 })dx } } \quad =\quad ln2\quad -\quad 2\quad +\cfrac { \pi }{ 2 } \quad \quad .\quad .\quad .\quad .\quad (1)\\ \\ \lambda =\int _{ 0 }^{ 1 } \frac { dx }{ 1+x^{ 3 } } \quad \\ \\ \lambda =\quad \int _{ 0 }^{ 1 } \frac { { x }^{ 2 }+1-{ x }^{ 2 }\quad }{ 1+x^{ 3 } } dx\quad \\ \quad =\quad \cfrac { 1 }{ 3 } \int _{ 0 }^{ 1 } \frac { 3{ x }^{ 2 }dx }{ 1+x^{ 3 } } \quad \quad +\quad \int _{ 0 }^{ 1 } \frac { (1+x)(1-x) }{ (1+x)({ x }^{ 2 }+1-x) } dx\\ \\ \quad ={ \quad \cfrac { 1 }{ 3 } [ln(1+x^{ 3 })] }_{ 0 }^{ 1 }\quad \quad +\quad \int _{ 0 }^{ 1 } \frac { 1-x }{ ({ x }^{ 2 }+1-x) } dx\\ \\ 3\lambda \quad =\quad \ln { 2 } \quad +\quad \cfrac { \pi }{ \sqrt { 3 } } \quad \quad ........\quad (2)\\ \\ \left\lceil 3\lambda +\ln { \rho } \right\rceil \quad =\left\lceil \ln { 2 } \quad +\quad \cfrac { \pi }{ \sqrt { 3 } } \quad +\quad ln2\quad -\quad 2\quad +\cfrac { \pi }{ 2 } \right\rceil \quad =\quad 3 .

This problem was disputed by several people, but they offered different values as the correct answer.

I previously updated the answer to 4, based on Deepanshu's initial solution.
After working through it, we realized that the answer should be 3, and the solution is being updated.

Sorry for the confusion which may have arisen from having multiple "The answer has been corrected" emails.

Calvin Lin Staff - 6 years, 7 months ago

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