While solving Real Integrals, do you obtain results in Complex Numbers?

Calculus Level 5

π / 4 π / 3 ( 1 tan x ( ln ( sin x ) ) + tan x ln ( cos x ) ) d x \large{ \int_{\pi/4}^{\pi/3} \left(\frac{1}{\tan x\ (\ln( \sin x))}+\frac{\tan x}{\ln (\cos x)}\right)\ \mathrm{d}x }

If the above integral equals to z z , z C z \in \mathbb C , where C \mathbb C denotes the set of complex numbers, evaluate the value of z |z| upto three correct places of decimals.


The answer is 1.572.

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1 solution

Satyajit Mohanty
Aug 24, 2015

π / 4 π / 3 ( 1 tan x ( ln sin x ) + tan x ln cos x ) d x \displaystyle \int_{\pi/4}^{\pi/3} \left(\frac{1}{\tan x\ (\ln \sin x)}+\frac{\tan x}{\ln \cos x}\right) \ dx

= π / 4 π / 3 ( ( ln ( ln sin x ) ) ( ln ( ln cos x ) ) ) d x = \displaystyle \int_{\pi/4}^{\pi/3} \bigg((\ln (\ln \sin x))'-(\ln (\ln \cos x))' \bigg)\ dx

= ( ln ( ln ( sin x ) ) ln ( ln ( cos x ) ) ) π / 4 π / 3 =\large{ \bigg(\ln(\ln(\sin x) )- \ln(\ln(\cos x)) \bigg) \Bigg|_{\pi/4}^{\pi/3} }

= ln ( ln ( 3 2 ) ) ln ( ln ( 1 2 ) ) = \ln \left( \ln \left(\dfrac{\sqrt{3}}{2} \right) \right) -\ln \left( \ln \left(\dfrac{1}{\sqrt{2}} \right) \right)

= ln [ ln ( 3 2 ) ln ( 1 2 ) ] 1.572 = \displaystyle\ln\left[\frac{\ln\left(\displaystyle\frac{\sqrt{3}}{2}\right)}{\ln\left(\displaystyle\frac{1}{2}\right)} \right] \approx \boxed{-1.572} and thus, it's modulus is 1.572 \boxed{1.572} .


Why did we suppose that the value of the integral might be a non-real (complex) entity? The answer is right below.

You can see that both

ln ( ln ( 3 2 ) ) \color{#D61F06}{\ln \left( \ln \left(\dfrac{\sqrt{3}}{2} \right) \right) \quad } and ln ( ln ( 1 2 ) ) \quad \color{#D61F06}{\ln \left( \ln \left(\dfrac{1}{\sqrt{2}} \right) \right) }

are non-real (complex) numbers, but then, we have

[ ln ( ln ( 3 2 ) ) ln ( ln ( 1 2 ) ) ] = ln [ ln ( 3 2 ) ln ( 1 2 ) ] \left[\color{#D61F06}{\ln \left( \ln \left(\dfrac{\sqrt{3}}{2} \right) \right) -\ln \left( \ln \left(\dfrac{1}{\sqrt{2}} \right) \right)} \right] = \displaystyle \color{#3D99F6}{ \ln \left [\frac{\ln \left( \displaystyle\frac{\sqrt{3}}{2}\right)}{\ln\left(\displaystyle\frac{1}{2}\right)} \right]}

as a real number.

@Satyajit Mohanty I don't think that your last explanation was the suitable ...

What if the integral was:

π 8 π 4 d x tan x ( ln ( 2 sin x ) ) ? ? \displaystyle \int_{\frac{\pi}{8}}^{\frac{\pi}{4}} \frac{dx}{\tan x (\ln (2\sin x)) } ??

Do you think the answer is complex?

Hasan Kassim - 5 years, 9 months ago

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Nopes. I'm not talking about generalizations. I'm talking about this particular integral.

Satyajit Mohanty - 5 years, 9 months ago

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Yes even your particular integral, it is real because the antiderivative of 1 x \frac{1}{x} is ln x \ln {\color{#D61F06}{|}} x {\color{#D61F06}{|}} .

Not because of the applied difference :)

Hasan Kassim - 5 years, 9 months ago

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