Do we need to calculate each and every term

Algebra Level 4

If ω 0 = 2015 ω_{0}=2015 and let 1 + ω n 1 ω n = ω n + 1 \frac{1+ω_{n}}{1-ω_{n}}=ω_{n+1} , then value if ω 2016 ω_{2016} is

ω 2011 ω_{2011} ω 2012 ω_{2012} ω 2010 ω_{2010} ω 2014 ω_{2014}

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3 solutions

Rohit Ner
Dec 12, 2015

w n + 1 = 1 + w n 1 w n = 1 + 1 + w n 1 1 w n 1 1 1 + w n 1 1 w n 1 = 1 w n 1 w 2016 = 1 w 2014 = 1 1 w 2012 = w 2012 \begin{aligned}{w}_{n+1}&=\frac{1+{w}_{n}}{1-{w}_{n}}\\&=\frac{1+\frac{1+{w}_{n-1}}{1-{w}_{n-1}}}{1-\frac{1+{w}_{n-1}}{1-{w}_{n-1}}}\\&=-\frac{1}{{w}_{n-1}}\\{w}_{2016}&=-\frac{1}{{w}_{2014}}\\&=-\frac{1}{-\frac{1}{{w}_{2012}}}\\&\huge\color{#3D99F6}{=\boxed{{w}_{2012}}}\end{aligned}

Aayush Patni
Jan 4, 2016

Just observe a simple recurrence

w(0) = 2015

w(1)= -1008/1007

w(2) = -1/2015

w(3)= 1007/1008

w(4)=2015

Thus for every w(n) where n=4k

we have w(n) = 2015

Thus w(2016)=w(2012)

Ravi Dwivedi
Dec 11, 2015

Let w n = tan ( x n ) w_{n}=\tan(x_{n}) for some x n R x_{n} \in R and n N n \in N .

This is because the range of tan x \tan x is R so any term of sequence(which consists of real terms) is equal to tan x \tan x for some x R x \in R

Then 1 + w n 1 w n = 1 + tan ( x n ) 1 tan ( x n ) = tan ( π 4 + x n ) = tan ( x n + 1 ) = w n + 1 \frac{1+w_{n}}{1-w_{n}}=\frac{1+\tan(x_{n})}{1-\tan(x_{n})}=\tan(\frac{\pi}{4}+x_{n})=\tan(x_{n+1})=w_{n+1}

So we get

w n + 2 = tan ( x n + 2 ) = tan ( π 2 + x n ) w_{n+2}=\tan(x_{n+2})=\tan(\frac{\pi}{2}+x_{n})

w n + 3 = tan ( x n + 3 ) = tan ( 3 π 4 + x n ) w_{n+3}= \tan(x_{n+3})=\tan(\frac{3\pi}{4}+x_{n})

w n + 4 = tan ( x n + 4 ) = tan ( π + x n ) = tan ( x n ) = w n w_{n+4}=\tan(x_{n+4})=\tan(\pi+x_{n})=\tan(x_{n})=w_{n}

We conclude that the sequence is periodic with period n = 4 n = 4 and hence answer is w 2016 = w 2012 w_{2016}=w_{2012}

Moderator note:

Since tan ( 2 π + θ ) = tan θ \tan ( 2 \pi + \theta ) = \tan \theta , it follows from the recurrence has a period of 4. How else can we know this?

Not 2 π 2\pi but π \pi

Ravi Dwivedi - 5 years, 5 months ago

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