Too small of an angle!

Geometry Level 3

Given tan ( x ) = 2 \tan(x) = 2 . If tan ( x 8 ) \large \tan \left ( \frac {x}{8} \right ) is the root of an 8 th 8^{\text{th}} degree monic polynomial with integer coefficients f ( x ) f(x) . What is the value of f ( 1 ) f(1) ?


The answer is 16.

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2 solutions

Vishal Sharma
Mar 27, 2014

We have one value of tan(x). Hence we'll get two values tan(x/2). Similarly 4 values tan(x/4) and ultimately 8 values of tan(x/8) Thus we have a degree 8 equation in tan(x/8) as variable whose roots are the eight values of tan(x/8). So now, replace tan(x/8) with 'x' and obtain similar equation in 'x' Lastly put x=1 and get the answer....

Shikhar Jaiswal
Mar 22, 2014

tan ( x ) = 2 \tan(x)=2

2 tan ( x 2 ) 1 tan 2 ( x 2 ) = 2 \Rightarrow \frac {2\tan(\frac {x}{2})}{1-\tan^2(\frac {x}{2})}=2

tan ( x 2 ) = 1 tan 2 ( x 2 ) \Rightarrow \tan (\frac {x}{2})=1-\tan^2(\frac {x}{2}) ......for simplicity..let tan ( x 4 ) = K \tan(\frac {x}{4})=K

2 K 1 K 2 = 1 4 K 2 ( 1 K 2 ) 2 \Rightarrow \frac {2K}{1-K^2}=1-\frac {4K^2}{(1-K^2)^2}

K 4 + 2 K 3 6 K 2 2 K + 1 = 0 \Rightarrow K^4+2K^3-6K^2-2K+1=0

again..let tan ( x 8 ) = L \tan (\frac {x}{8})=L ..we get K = 2 L 1 L 2 K=\frac {2L}{1-L^2}

also....

( 2 L 1 L 2 ) 4 + 2 ( 2 L 1 L 2 ) 3 6 ( 2 L 1 L 2 ) 2 2 ( 2 L 1 L 2 ) + 1 = 0 (\frac {2L}{1-L^2})^4+2(\frac {2L}{1-L^2})^3-6(\frac {2L}{1-L^2})^2-2(\frac {2L}{1-L^2})+1=0

( 2 L ) 4 + 2 ( 2 L ) 3 ( 1 L 2 ) 6 ( 2 L ) 2 ( 1 L 2 ) 2 ( 2 L ) ( 1 L 2 ) 3 + ( 1 L 2 ) 4 = 0 \Rightarrow (2L)^4+2(2L)^3(1-L^2)-6(2L)^2(1-L^2)-2(2L)(1-L^2)^3+(1-L^2)^4=0

Clearly we can see a MONIC POLYNOMIAL with tan ( x 8 ) \tan (\frac {x}{8}) as a ROOT

f ( 1 ) = 16 \boxed{f(1)=16}

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