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Calculus Level 5

5 6 ln ( 11 x + 12 ) x 2 + 42 d x = ln ( A ) B C × tan 1 ( C D ) \large\displaystyle\int \limits^{6}_{5}\frac{\ln\left( 11x+ 12\right) }{x^{2}+ 42} \, dx = \frac {\ln (A)}{B \sqrt{C}} \times \tan^{-1}\bigg( \frac {\sqrt{C}}{D} \bigg)

Suppose the equation above is true for constants A , B , C A,B,C and D D , what is the value of A + B + C + D A+B+C+D ?


The answer is 5342.

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1 solution

Let u = 11 x + 12 \displaystyle u=11x+12 , this turns our integral to

67 78 11 ln ( u ) u 2 12 u + 5226 d u \displaystyle \int_{67}^{78} 11 \dfrac{\ln(u)}{u^{2}-12u+5226} du

Now, let x = 5226 u \displaystyle x=\dfrac{5226}{u} , we will get

67 78 11 ln ( u ) u 2 12 u + 5226 d u = 11 ln ( 5226 ) 2 67 78 1 u 2 12 u + 5226 d u \displaystyle \int_{67}^{78} 11 \dfrac{\ln(u)}{u^{2}-12u+5226} du=\dfrac{11\ln(5226)}{2}\int_{67}^{78} \dfrac{1}{u^{2}-12u+5226} du

The latter integral is now easy to find,

1 5082 arctan ( x 12 5082 ) \displaystyle \dfrac{1}{\sqrt{5082}} \arctan(\dfrac{x-12}{\sqrt{5082}})

Substituting jn the boundary of integration,

arctan ( 6 42 ) arctan ( 5 42 ) = arctan ( 42 72 ) \displaystyle \arctan(\dfrac{6}{\sqrt{42}})- \arctan(\dfrac{5}{\sqrt{42}})= \arctan(\dfrac{\sqrt{42}}{72})

Plugging all these in to get,

ln ( 5226 ) 2 42 arctan ( 42 72 ) \displaystyle \dfrac{\ln(5226)}{2\sqrt{42}} \arctan(\dfrac{\sqrt{42}}{72})

So, the sum is then 5226 + 2 + 42 + 72 = 5342 \displaystyle 5226+2+42+72= \boxed{5342}

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