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Calculus Level 5

I = e π 6 e π sin ( log ( sin ( log ( x ) ) ) ) cos ( log ( x ) ) x sin ( log ( x ) ) d x \large {I = \int^{\sqrt{e}^{\pi}}_{e^{\frac{\pi}{6}}} {\dfrac{\sin{(\log{(\sin{(\log{(x)})})})}\cos{(\log{(x)})}} {x \sin{(\log{(x)})}}} \, dx}

If I I can be expressed in the form A + cos ( B ) A+\cos{(B)} , where A A and B B are real numbers, A A is a negative integer, and B B is the smallest possible positive number that satisfies the value of I I , find I + A + B I+A+B to 2 decimal places.


NOTE:

log ( x ) \log{(x)} denotes the natural logarithm of x x .


The answer is -0.54.

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1 solution

Akeel Howell
Jan 8, 2017

Relevant wiki: u u -Substitution

I = e 6 π e π sin ( log ( sin ( log ( x ) ) ) ) cos ( log ( x ) ) x sin ( log ( x ) ) d x \LARGE{I = \int^{\sqrt{e}^{\pi}}_{\sqrt[6]{e}^{\pi}} {\dfrac{\sin{(\log{(\sin{(\log{(x)})})})}\cos{(\log{(x)})}} {x \sin{(\log{(x)})}}} dx}

We can use a substitution to simplify the integrand. We see this instantly if we set u = log ( sin ( log ( x ) ) ) u = \log{(\sin{(\log{(x)})})}\\ . This gives that d u = cos ( log ( x ) ) x sin ( log ( x ) ) d x . du = \dfrac{\cos{(\log{(x)})}}{x\sin{(\log{(x)})}}dx.


This leaves us with I = log 2 0 sin ( u ) d u = cos ( u ) \large{\displaystyle{I = \int^{0}_{-\log{2}} {\sin{(u)}du}} = -\cos{(u)}} . Substituting again to have u \large{u} in terms of x \large{x} , we see that I = cos ( log ( sin ( log ( x ) ) ) ) e 6 π e π \displaystyle{\large{I = -\cos{(\log{(\sin{(\log{(x)})})})}}|^{\sqrt{e^{\pi}}}_{\sqrt[6]{e}^{\pi}}} So I = 1 + cos ( log ( 2 ) ) I = -1+\cos{(\log{(2)})}\\ Thus, A = 1 A=-1 and B = log ( 2 ) B=\log{(2)}\\ . I + A + B = 1 + cos ( log ( 2 ) ) 1 + log ( 2 ) 0.54 \therefore I+A+B = -1+\cos{(\log{(2)})}-1+\log{(2)} \approx \boxed{-0.54} ( 2 (2 decimal places ) )

Limits on the integral are wrong, aside from that, nice work. :) Same here.

A Former Brilliant Member - 4 years, 4 months ago

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Oh sorry. I just copied the same limits on the previous integral. Fixed it.

Akeel Howell - 4 years, 4 months ago

A nice desinged easy sum. Good one! +1 from me!

Md Zuhair - 4 years, 5 months ago

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Thank you!

Akeel Howell - 4 years, 5 months ago

Hey @Akeel Howell you can keep the lower limit as e 6 π \sqrt[6]{e}^{\pi}

Md Zuhair - 4 years, 4 months ago

the answer would be , acc. to me , -2+cos(ln2))-ln2 instead of + , check it again , the exact value of B B i get is ln(1/2) , pl check it .

A Former Brilliant Member - 4 years, 4 months ago

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