# limiting value of Cosine

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• June 27th 2010, 07:29 PM
Chandru1
limiting value of Cosine
Hi--

Please help me as to what value does $\cos(\theta) \times \cos(\theta/2) \times \cdots \times \cos(\theta/2^{n})$ approach as $n \to \infty$.
• June 27th 2010, 07:41 PM
Also sprach Zarathustra
Hint:

Multiply (and divide) by $2sin\frac{\theta}{2}$ and use:

$sin2x=2sinxcosx$
• June 27th 2010, 08:41 PM
Chandru1
I was solving an analysis problem and i need this quantity to have 1/2^{n} multiplied by some value. If this doesn't work out then my claim would be incorrect.
• June 28th 2010, 02:24 AM
mr fantastic
Quote:

Originally Posted by Chandru1
Hi--

Please help me as to what value does $\cos(\theta) \times \cos(\theta/2) \times \cdots \times \cos(\theta/2^{n})$ approach as $n \to \infty$.

This infinite product is based on a famous infinite product first discovered by Euler:

$\sin (x) = 2 \sin \left(\frac{x}{2}\right) \cos \left(\frac{x}{2}\right)$

$= 2^2 \sin \left(\frac{x}{4}\right) \cos \left(\frac{x}{4}\right) \cos \left(\frac{x}{2}\right)$

$= 2^3 \sin \left(\frac{x}{8}\right) \cos \left(\frac{x}{8}\right) \cos \left(\frac{x}{4}\right) \cos \left(\frac{x}{2}\right)$

= ....

$= 2^n \sin \left(\frac{x}{2^n}\right) \cos \left(\frac{x}{2^n}\right) .... \cos \left(\frac{x}{2}\right)$

$= x \cdot \left[ \frac{ \sin \left(\frac{x}{2^n}\right) }{\frac{x}{2^n}} \right] \cos \left(\frac{x}{2}\right) \cos \left(\frac{x}{4}\right).... \cos \left(\frac{x}{2^n}\right)$.

Now take the limit $n \to \infty$:

$\sin (x) = x \cos \left(\frac{x}{2}\right) \cos \left(\frac{x}{4}\right).... \cos \left(\frac{x}{2^n}\right) ....$

I hope you can see what to do with this famous result.
• June 28th 2010, 03:45 AM
Chandru1
thanks
Mr. Fantastic thanks for the fantastic answer.