Given tan ( x ) = 2 . If tan ( 8 x ) is the root of an 8 th degree monic polynomial with integer coefficients f ( x ) . What is the value of f ( 1 ) ?
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tan ( x ) = 2
⇒ 1 − tan 2 ( 2 x ) 2 tan ( 2 x ) = 2
⇒ tan ( 2 x ) = 1 − tan 2 ( 2 x ) ......for simplicity..let tan ( 4 x ) = K
⇒ 1 − K 2 2 K = 1 − ( 1 − K 2 ) 2 4 K 2
⇒ K 4 + 2 K 3 − 6 K 2 − 2 K + 1 = 0
again..let tan ( 8 x ) = L ..we get K = 1 − L 2 2 L
also....
( 1 − L 2 2 L ) 4 + 2 ( 1 − L 2 2 L ) 3 − 6 ( 1 − L 2 2 L ) 2 − 2 ( 1 − L 2 2 L ) + 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
Clearly we can see a MONIC POLYNOMIAL with tan ( 8 x ) as a ROOT
f ( 1 ) = 1 6
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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....