The polynomial f ( x ) = x 2 0 0 7 + 1 7 x 2 0 0 6 + 1 has distinct roots r 1 , r 2 , … , r 2 0 0 7 . A polynomial P of degree 2007 has the property that P ( r j + r j 1 ) = 0 for j = 1 , 2 , … , 2 0 0 7 .
Compute 1 7 2 5 9 × P ( − 1 ) P ( 1 ) .
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same here. nice problem.
i was doing same but didnt notice substitution of w
Can I ask its source?
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We can see that none of the roots of f ( x ) is zero. So we can form a polynomial whose roots are the numbers r j 1 for j = 1 , 2 , 3 , . . . , 2 0 0 7 and with 1 as leading coefficient just by considering g ( x ) = x 2 0 0 7 f ( x 1 ) , and, additionally, g ( x ) = ∏ j = 1 2 0 0 7 ( x − r j 1 ) .
Since P ( x ) = c ∏ j = 1 2 0 0 7 ( x − r j − r j 1 ) where c is a constant then, P ( x + x 1 ) = c j = 1 ∏ 2 0 0 7 ( x + x 1 − r j − r j 1 ) = c j = 1 ∏ 2 0 0 7 x ( x − r j ) ( x − r j 1 ) = c x − 2 0 0 7 f ( x ) g ( x ) = c f ( x ) f ( x 1 ) .
Now, since e i 3 π + e i 3 π 1 = 1 , e i 3 2 π + e i 3 2 π 1 = − 1 , and using the equality above, we obtain that 1 7 2 5 9 × P ( − 1 ) P ( 1 ) = 1 7 2 5 9 × P ( e i 3 2 π + e i 3 2 π 1 ) P ( e i 3 π + e i 3 π 1 ) = 1 7 2 5 9 × f ( e i 3 2 π ) f ( e i 3 2 π 1 ) f ( i 3 π ) f ( e i 3 π 1 ) = 1 7 2 5 9 × 2 5 9 1 7 2 = 1 7 .