Can you generalize it?

Calculus Level 5

lim x 0 e x π x sin 2015 ( t ) t 2016 d t = ? \large \lim_{x\to 0}\int\limits_{ex}^{\pi x} \dfrac{\sin^{2015}(t)}{t^{2016}}\, dt= \ ?

Submit your answer to 3 decimal places.


The answer is 0.145.

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1 solution

Hasan Kassim
Aug 16, 2015

L = lim x 0 a x b x sin n t t n + 1 d t \displaystyle L =\lim_{x\to 0} \int_{ax}^{bx} \frac{\sin^nt}{t^{n+1}} dt

Use the substitution: u = t x , d u = 1 x d t u=\frac{t}{x} , du = \frac{1}{x}dt :

L = lim x 0 a b sin n ( x u ) ( x u ) n + 1 x d u \displaystyle L= \lim_{x\to 0} \int_{a}^{b} \frac{\sin^n(xu)}{(xu)^{n+1}} xdu

= lim x 0 a b sin n ( x u ) ( x u ) n d u u \displaystyle = \lim_{x\to 0} \int_{a}^{b} \frac{\sin^n(xu)}{(xu)^n} \frac{du}{u}

= a b ( lim x 0 sin n ( x u ) ( x u ) n ) d u u \displaystyle = \int_{a}^{b} \bigg(\lim_{x\to 0} \frac{\sin^n(xu)}{(xu)^n} \bigg) \frac{du}{u}

= a b ( 1 n ) d u u \displaystyle = \int_{a}^{b} (1^n) \frac{du}{u}

= ln u a b = ln b a \displaystyle = \ln |u| |_{a}^{b} = \boxed{\ln |\frac{b}{a}| }

For this problem, evaluate with a = e , b = π a= e , b=\pi to get the value 0.145 0.145 to 3 3 decimal places.

Moderator note:

Note that t t is a variable and not a constant. Thus, you will need to explain what you mean by u = t x u = \frac{t}{x} .

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