Tessellate S.T.E.M.S (2019) - Mathematics - Category C - Set 3 - Objective Problem 4

Calculus Level 3

Let f ( x ) = sin ( x ) x f(x) = \dfrac{\sin(x)}{x} . Find lim T 1 T 0 T 1 + f ( x ) 2 d x . \lim_{T\to\infty}\frac{1}{T}\int_0^T\sqrt{1+f'(x)^2}dx.

0 1 \infty does not exist

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1 solution

Tom Engelsman
Nov 13, 2018

Let g ( T ) = 0 T 1 + ( f ( x ) ) 2 d x g(T) = \int_{0}^{T} \sqrt{1 + (f'(x))^2} dx and h ( T ) = T h(T) = T . The former function is just the arc length of f ( x ) = s i n ( x ) x f(x) = \frac{sin(x)}{x} for the entire right half of the curve, which is the length over the entire positive reals as T tends toward infinity. Upon first observation:

l i m T g ( T ) h ( T ) = lim_{T \rightarrow \infty} \frac{g(T)}{h(T)} = \frac{\infty}{\infty}

and applying L'Hopital's Rule yields:

l i m T g ( T ) h ( T ) = 1 + ( f ( T ) 2 / 1 = 1 + ( T c o s ( T ) s i n ( T ) T 2 ) 2 = 1 + ( c o s ( T ) T s i n ( T ) T 2 ) 2 = 1 = 1 . lim_{T \rightarrow \infty} \frac{g'(T)}{h'(T)} = \sqrt{1 + (f'(T)^2} / 1 = \sqrt{1 + (\frac{T \cdot cos(T) - sin(T)}{T^2})^2} = \sqrt{1 + (\frac{cos(T)}{T} -\frac{sin(T)}{T^2})^2} = \sqrt{1} = \boxed{1}.

1 = ± 1 \sqrt { 1 } = \pm 1 .

. . - 3 months, 3 weeks ago

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