Just for fun!

Calculus Level 4

Given that

{ f ( k , x ) = ( e x ln k + e k ln x + e k ln k ) d x f ( e , 0 ) = 1 f ( 4 , 3 ) = a b + c ln 2 \begin{cases} f(k,x)=\displaystyle \int \left(e^{x \ln k}+e^{k \ln x}+e^{k \ln k}\right) dx \\ f(e,0)=1 \\ f(4,3)=\dfrac{a}{b}+\dfrac{c}{\ln 2} \end{cases}

Find a 25 b c a-25 bc , where a a , b b and c c are positive integers with a a and b b being coprime integers.


The answer is 83.

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2 solutions

Pranjal Jain
Dec 8, 2014

f ( a , x ) = ( a x + x a + a a ) d x f(a,x)=\displaystyle\int (a^{x}+x^{a}+a^{a}) dx

f ( a , x ) = a x l n a + x a + 1 a + 1 + a a x + c f(a,x)=\dfrac{a^{x}}{ln\ a}+\dfrac{x^{a+1}}{a+1}+a^{a}x+c

Substituting a = e , x = 0 c = 0 a=e,\ x=0\ \Rightarrow c=0

Substituting a = 4 , x = 3 a=4,\ x=3 ,

f ( 4 , 3 ) = 4083 5 + 32 l n 2 f(4,3)=\dfrac{4083}{5}+\dfrac{32}{ln\ 2}

a 25 2 b c d = 4083 25 2 × 5 × 32 × 2 = 83 \Rightarrow a-\frac{25}{2}bcd=4083-\frac{25}{2}×5×32×2=\boxed{83}

seriously!! a level 4 problem??

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It was Level 2 when I posted it. God knows what happened. (The title is the proof for my words.)

Pranjal Jain - 6 years ago

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yeah u r right!

Chew-Seong Cheong
Aug 26, 2018

f ( k , x ) = ( e x ln k + e k ln x + e k ln k ) d x = ( k x + x k + k k ) d x = k x ln k + x k + 1 k + 1 + k k x + C where C is the constant of integration. f ( e , 0 ) = 1 1 + 0 + 0 + C = 1 C = 0 f ( k , x ) = k x ln k + x k + 1 k + 1 + k k x f ( 4 , 3 ) = 4 3 ln 4 + 3 4 + 1 4 + 1 + 4 4 × 3 = 64 2 ln 2 + 243 5 + 768 = 4083 5 + 32 ln 2 \begin{aligned} f(k,x) & = \int \left(e^{x\ln k} + e^{k\ln x} + e^{k\ln k}\right) dx \\ & = \int \left(k^x + x^k + k^k \right) dx \\ & = \frac {k^x}{\ln k} + \frac {x^{k+1}}{k+1} + k^k x + \color{#3D99F6} C & \small \color{#3D99F6} \text{where }C \text{ is the constant of integration.} \\ f(e, 0) & = \frac 11 + 0 + 0 + C = 1 & \small \color{#3D99F6} \implies C = 0 \\ \implies f(k,x) & = \frac {k^x}{\ln k} + \frac {x^{k+1}}{k+1} + k^k x \\ f(4,3) & = \frac {4^3}{\ln 4} + \frac {3^{4+1}}{4+1} + 4^4 \times 3 \\ & = \frac {64}{2\ln 2} + \frac {243}5 + 768 \\ & = \frac {4083}5 + \frac {32}{\ln 2} \end{aligned}

Therefore, a 25 b c = 4083 25 ( 5 ) ( 32 ) = 83 a-25bc = 4083 - 25(5)(32) = \boxed{83} .

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