Logs and graphs

Calculus Level 4

Let the area bounded by the equation : ln y + ln 2 x = 1 \ln y+\ln^{2}x=1 and the x x -axis be A . A.

Find the positive integer a , b , c a,b,c satisfying A 4 = a e b ζ ( c ) . A^4 = a e^{b}\zeta (c) .

Submit a × c 2 × b a\times c - 2\times b

Notation: ζ ( ) \zeta(\cdot) denotes the Riemann Zeta function .


The answer is 2.

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2 solutions

Dwaipayan Shikari
Apr 10, 2021

log ( y ) + log 2 ( x ) = 1 y = e 1 log 2 ( x ) \log(y)+\log^2(x) = 1 \implies y= e^{1-\log^2(x)}

y = 0 y=0 when it touches x x axis . So there are two limits x x\rightarrow{∞} and x 0 x\rightarrow{0}

Area = 0 e 1 log 2 ( x ) d x \displaystyle\int_0^∞ e^{1-\log^2(x)} dx Take log ( x ) = t \log(x)=t , the integral becomes= e e t t 2 d t \displaystyle e\int_{-∞}^{∞} e^{t-t^2} dt = e e ( t 1 2 ) 2 + 1 4 d t = e\int_{-∞}^∞ e^{-(t-\frac{1}{2})^2+\frac{1}{4}}dt = e 5 4 e ( t 1 2 ) 2 d t = e 5 4 π (Gaussian Integral) = e^{\frac{5}{4}} \int_{-∞}^∞ e^{-(t-\frac{1}{2})^2} dt = e^{\frac{5}{4}} \sqrt{π} \textrm{(Gaussian Integral)} So A = e 5 4 π A= e^{\frac{5}{4}} \sqrt{π} and A 4 = e 5 π 2 = ( 2 2 + 2 ) e 2 2 + 1 π 2 6 A^4 = e^5 π^2 = (2^2+2)e^{2^2+1}\dfrac{π^2}{6}

So A 4 = ( 2 2 + 2 ) e 2 2 + 1 ζ ( 2 ) A^4 = (2^2+2) e^{2^2+1} \zeta(2) - Answer b = 2 \boxed{b=2}

Veselin Dimov
Apr 13, 2021

To be honest, it's rather easy to guess b = 2 b=2 . Maybe you can give it like: A 4 = a e b ζ ( c ) A^4=ae^b\zeta(c) And ask a + b + c = ? a+b+c=? . What do you think, @Zakir Husain ?

Changed the Question

Zakir Husain - 2 months ago

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