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Calculus Level 5

0 π / 2 x 2 sin x d x = A π G B C ζ ( D ) \large \int _{ 0 }^{ \pi /2 }{ \dfrac { { x }^{ 2 } }{ \sin { x } } \, dx } =A\pi G-\dfrac { B }{ C } \zeta (D)

If the equation above holds true for positive integers A , B , C A,B,C and D D , with B , C B,C coprime, find A + B + C + D A+B+C+D .

Notations :

  • G G denotes the Catalan's constant , G = n = 0 ( 1 ) n ( 2 n + 1 ) 2 0.916 \displaystyle G = \sum_{n=0}^\infty \dfrac{ (-1)^n}{(2n+1)^2} \approx 0.916 .

  • ζ ( ) \zeta(\cdot) denotes the Riemann zeta function .


The answer is 14.

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1 solution

Let I = 0 π 2 x 2 csc x d x \displaystyle I = \int_{0}^{\frac{\pi}{2}} x^2 \csc x dx

Applying IBP ,

I = 2 0 π 2 x ln c o s e c x c o t x d x = 0 π 2 2 x [ ln ( 2 sin 2 x 2 ) ln ( s i n x ) ] d x \displaystyle I = -2\int_{0}^{\frac{\pi}{2}} x\ln|cosecx-cotx|dx = \int_{0}^{\frac{\pi}{2}} -2x[\ln(2\sin^2\frac{x}{2}) - \ln(sinx)]dx

Splitting into parts we get ,

I = 2 0 π 2 x ln ( 2 sin 2 x 2 ) d x + 2 0 π 2 x ln ( sin x ) d x \displaystyle I = -2\int_{0}^{\frac{\pi}{2}} x\ln(2\sin^2\frac{x}{2}) dx + 2\int_{0}^{\frac{\pi}{2}} x\ln(\sin x) dx

Both the integrals can be calculated separately and I have done that in previous cases so I'm skipping the part,

Consider a function F ( a , b ) = 0 π 2 sin a x cos b x d x = 2 a Γ ( a + 1 ) π cos ( b π 2 ) Γ ( a + b + 2 2 ) Γ ( a b + 2 2 ) \displaystyle F(a,b) = \int_{0}^{\frac{\pi}{2}} \sin^a x \cos bx dx = \frac{2^{-a}\Gamma(a+1)\pi \cos(\frac{b\pi}{2})}{\Gamma(\frac{a+b+2}{2})\Gamma(\frac{a-b+2}{2})}

We can calculate 2 F ( 1 , 1 ) a b \displaystyle \frac{\partial^2 F(1,1)}{\partial a \partial b} which equals our integrals .

After calculation , { 2 0 π 2 x ln ( 2 sin 2 x 2 ) d x = 2 π G + 35 8 ζ ( 3 ) π 2 4 ln 2 2 0 π 2 x ln ( sin x ) d x = 7 8 ζ ( 3 ) π 2 4 ln 2 \displaystyle \begin{cases} 2\int_{0}^{\frac{\pi}{2}} x\ln(2\sin^2\frac{x}{2}) dx = -2\pi G + \frac{35}{8}\zeta(3) - \frac{\pi^2}{4}\ln 2 \\ 2\int_{0}^{\frac{\pi}{2}} x\ln(\sin x) dx = \frac{7}{8}\zeta(3)-\frac{\pi^2}{4}\ln 2 \end{cases}

So I = 2 π G 7 2 ζ ( 3 ) \displaystyle I = 2\pi G-\frac{7}{2}\zeta(3) , Thus answer is 2 + 7 + 2 + 3 = 14 \boxed{2+7+2+3=14}

what is IBP?

gon furikusu - 2 years, 11 months ago

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Integration by parts

Prithwish Mukherjee - 1 year, 10 months ago

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