Crazy Limits!!! 2

Calculus Level 5

Let

α = lim m lim n [ cos ( n ! π x ) ] 2 m \displaystyle \alpha = \lim_{m \to \infty} \lim_{n \to \infty } [\cos(n! \pi x)]^{2m}

When x is rational

β = lim m lim n [ c o s ( n ! π x ) ] 2 m \displaystyle \beta = \lim_{m \to \infty }\lim_{n \to \infty } [cos(n! \pi x)]^{2m}

When x is irrational

Then the area of the triangle formed by vertices ( α , β ) (\alpha,\beta) , ( π e , π (\pi^{e}, -\pi ), ( e π , e (e^{\pi}, e ) is A

Then find 'A + 3'

Also try Crazy Limits!!! 1

Details

  • answer to nearest integer


The answer is 67.

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1 solution

Krishna Sharma
Oct 19, 2014

First of all we will solve α \alpha

Now

Let x = p q \frac{p}{q}

Substituting in α \alpha

α = lim m lim n [ c o s ( n ! π p q ) ] 2 m \displaystyle \alpha = \lim_{m \to \infty} \lim_{n \to \infty} [cos(n!\pi \frac{p}{q})]^{2m}

Now we observer that

q will cancel out 100% (with one term of n!)

Then we will get

Cos as multiple of π \pi

And we know that

c o s ( n π ) = ± 1 \displaystyle cos(n\pi) = \pm1

But the power is 2m, taking 2 inside we will only get

c o s ( w h a t e v e r ) = 1 cos(whatever) = 1

And 1 = 1 1^{\infty} = 1

We finally get

α = 1 \displaystyle \boxed{\alpha = 1}

Now solving for β \beta

As x is irrational let is be x

β = lim m lim n [ c o s ( n ! π x ) ] 2 m \displaystyle \beta = \lim_{m \to \infty} \lim_{n \to \infty} [cos(n!\pi x)]^{2m}

Now

Cos(whatever) will oscillate b/w ± 1 \pm 1

As the power is 2m, taking 2 inside

Cos(whatever) will oscillate b/w

0 and 1

Let angle of cos be 'k'

lim m [ c o s ( k ) ] m = 0 \displaystyle \lim_{m \to \infty} [cos(k)]^{m} = 0

Hence we get

α = 1 , β = 0 \alpha = 1, \beta = 0

Finding the area will require calculator which you can do (comes around 63.94)

Final answer comes 67

Are you implying that they cheated?

Anastasiya Romanova - 6 years, 7 months ago

i inserted 66, thinking you wanted the floor ;; _ _;;;

Aritra Jana - 6 years, 4 months ago

Thanks. Given the edits to your question, those who previously answered 63, 64, 66 have been marked correct. The correct answer is now 67.

Calvin Lin Staff - 6 years, 4 months ago

An additional comment: Note that when x x is rational, we can always take n n large enough so that the argument of cos \cos is an even multiple of π \pi , thus leading to the limit lim n cos ( n ! π x ) = 1 \lim_{n\to \infty }\cos (n! \pi x)=1 . Similarly, when x x is irrational, no matter how large n n is n ! x n! x can never be of the form k π k\pi , leading to values of cos ( n ! π x ) \cos (n!\pi x) lying in the range 1 , 1 -1,1 ,. So, the use of 2 m 2m is actually not needed here and the same answer can be obtained if 2 m 2m is replaced by m m .

Samrat Mukhopadhyay - 4 years, 9 months ago

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