Typical Indefinite Integration 2

Calculus Level 3

ln ( x ) ln ( x ) [ ln ( x ) ( ln ( ln ( x ) ) + 1 ) + 1 ] x d x { \displaystyle \int \dfrac{\ln(x)^{\ln(x)}\left [\ln(x)\left (\ln(\ln(x))+1 \right )+1\right]}{x} \, dx}

Ignoring the constant of integration, if the integral above can be expressed as ( f ( x ) ) f ( x ) + 1 {(f(x))^{f(x)+1}} , then find the value of f ( e ) f(e) .


The answer is 1.000.

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2 solutions

Kenny Lau
Jul 24, 2015

ln ( x ) ln ( x ) [ ln ( x ) ( ln ( ln ( x ) ) + 1 ) + 1 ] x d x \displaystyle \int \dfrac{\ln(x)^{\ln(x)}\left [\ln(x)\left (\ln(\ln(x))+1 \right )+1\right]}{x} \, \mathrm dx

= ( ln ( x ) ln ( x ) [ ln ( x ) ( ln ( ln ( x ) ) + 1 ) + 1 ] ) 1 x d x \displaystyle \int (\ln(x)^{\ln(x)}\left [\ln(x)\left (\ln(\ln(x))+1 \right )+1\right])\frac1x \, \mathrm dx

Let u = ln ( x ) u=\ln(x) , then d u = 1 x d x \mathrm du=\frac1x\mathrm dx .

= u u [ u ( ln ( u ) + 1 ) + 1 ] d u \displaystyle \int u^u[u(\ln(u)+1)+1]\,\mathrm du

= u u u ( ln ( u ) + 1 ) d u + u u d u \displaystyle \int u^uu(\ln(u)+1)\,\mathrm du + \int u^u\,\mathrm du

Let y = u u y=u^u , then d y = u u ( ln ( u ) + 1 ) d u \mathrm dy=u^u(\ln(u)+1)\mathrm du .

= u d y + y d u \displaystyle \int u \,\mathrm dy + \int y\,\mathrm du

Integration by parts.

= u y y d u + y d u \displaystyle uy - \int y \,\mathrm du + \int y\,\mathrm du

= u y \displaystyle uy

= u × u u \displaystyle u\times u^u

= u u + 1 \displaystyle u^{u+1}

= ln ( x ) ln ( x ) + 1 \displaystyle \ln(x)^{\ln(x)+1}

@Kenny Lau How ln ( x ) ln ( x ) + 1 can be equal to 1 ? \text { How } \displaystyle \ln ( x ) ^ { \ln ( x ) + 1 } \text { can be equal to } \boxed { 1 } \text { ? }

. . - 3 months, 2 weeks ago
Satyajit Mohanty
Jul 22, 2015

Therefore, f ( x ) = ln ( x ) f(x) = \ln(x) and so, our answer turns out to be 1 \boxed{1}

Since there's ln ( x ) \ln(x) everywhere, it's better to let x = e y x = e^y then use the fact that ( y y ) = y y ( ln ( y ) + 1 ) (y^y)' = y^y (\ln(y) +1) . The answer should be immediate afterwards.

Pi Han Goh - 5 years, 10 months ago

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