Let a 1 0 th degree polynomial f ( x ) have all the positive integers from 1 to 10 inclusive as its roots. If f ( 1 1 ) = 1 1 , what is the value of f ( 1 2 ) ?
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Practically the same solution as you : )
Since all the roots of the polynomial are known, f ( x ) can be written as f ( x ) = k ∏ i = 1 1 0 ( x − i ) .
Evaluating f ( 1 1 ) , we get the relation f ( 1 1 ) = k × 1 0 ! = 1 1 , which implies k = 1 0 ! 1 1 .
Simple substitution would now give f ( 1 2 ) = k ∏ i = 1 1 0 ( 1 2 − i ) = 1 0 ! 1 1 1 1 ! = 1 1 2 = 1 2 1 .
f ( x ) has roots 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 1 0 . Therefore,
f ( x ) = k ( x − 1 ) ( x − 2 ) ( x − 3 ) ( x − 4 ) ( x − 5 ) ( x − 6 ) ( x − 7 ) ( x − 8 ) ( x − 9 ) ( x − 1 0 )
f ( 1 1 ) = 1 1 k ( 1 1 − 1 ) ( 1 1 − 2 ) ( 1 1 − 3 ) ( 1 1 − 4 ) ( 1 1 − 5 ) ( 1 1 − 6 ) ( 1 1 − 7 ) ( 1 1 − 8 ) ( 1 1 − 9 ) ( 1 1 − 1 0 ) = 1 1 k ( 1 0 ) ( 9 ) ( 8 ) ( 7 ) ( 6 ) ( 5 ) ( 4 ) ( 3 ) ( 2 ) ( 1 ) = 1 1 ⟹ k = 1 0 ! 1 1
We then calculate f ( 1 2 ) :
f ( 1 2 ) = 1 0 ! 1 1 ( 1 2 − 1 ) ( 1 2 − 2 ) ( 1 2 − 3 ) ( 1 2 − 4 ) ( 1 2 − 5 ) ( 1 2 − 6 ) ( 1 2 − 7 ) ( 1 2 − 8 ) ( 1 2 − 9 ) ( 1 2 − 1 0 ) = 1 0 ! 1 1 ( 1 1 ) ( 1 0 ) ( 9 ) ( 8 ) ( 7 ) ( 6 ) ( 5 ) ( 4 ) ( 3 ) ( 2 ) = 1 0 ! 1 1 2 ( 1 0 ! ) = 1 2 1
The real purpose of me writing this solution:
To tell you that
f ( x ) = 3 6 2 8 8 0 0 1 1 x 1 0 − 7 2 5 7 6 0 1 2 1 x 9 + 3 0 2 4 0 1 2 1 x 8 − 2 4 1 9 2 1 3 3 1 x 7 + 1 7 2 8 0 0 8 2 6 4 3 x 6 − 3 4 5 6 0 9 4 5 0 1 x 5 + 3 6 2 8 8 0 3 7 5 8 6 2 3 x 4 − 3 6 2 8 8 9 2 5 0 4 5 x 3 + 5 0 4 0 0 1 9 4 8 4 6 3 x 2 − 2 5 2 0 8 1 1 9 1 x + 1 1
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f ( x ) = A r = 1 ∏ 1 0 ( x − r )
⟹ f ( 1 1 ) f ( 1 2 ) = A r = 1 ∏ 1 0 ( 1 1 − r ) A r = 1 ∏ 1 0 ( 1 2 − r ) = 1 0 ! 1 1 ! = 1 1
⟹ f ( 1 2 ) = 1 1 ⋅ f ( 1 1 ) = 1 1 ⋅ 1 1 = 1 2 1