10 degrees above Zero

Algebra Level 4

Let a 1 0 th 10^\text{th} degree polynomial f ( x ) f(x) have all the positive integers from 1 to 10 inclusive as its roots. If f ( 11 ) = 11 f(11)=11 , what is the value of f ( 12 ) f(12) ?


The answer is 121.

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3 solutions

Rishabh Jain
Jul 13, 2016

f ( x ) = A r = 1 10 ( x r ) f(x)=\color{#D61F06}{A}\prod_{r=1}^{10}(x-r)

f ( 12 ) f ( 11 ) = A r = 1 10 ( 12 r ) A r = 1 10 ( 11 r ) = 11 ! 10 ! = 11 \implies \dfrac{f(12)}{f(11)}=\dfrac{\cancel{\color{#D61F06}{A}}\displaystyle\prod_{r=1}^{10}(12-r)}{\cancel{\color{#D61F06}{A}}\displaystyle\prod_{r=1}^{10}(11-r)}=\dfrac{11\cancel{!}}{\cancel{10!}}=11

f ( 12 ) = 11 f ( 11 ) = 11 11 = 121 \implies f(12)=11\cdot f(11)=11\cdot 11=\boxed{121}

Practically the same solution as you : ) :)

Michael Fuller - 4 years, 11 months ago

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That's great... :-)

Rishabh Jain - 4 years, 11 months ago

Since all the roots of the polynomial are known, f ( x ) f(x) can be written as f ( x ) = k i = 1 10 ( x i ) f(x)=k\prod_{i=1}^{10}(x-i) .

Evaluating f ( 11 ) f(11) , we get the relation f ( 11 ) = k × 10 ! = 11 f(11)=k\times 10! = 11 , which implies k = 11 10 ! k=\frac{11}{10!} .

Simple substitution would now give f ( 12 ) = k i = 1 10 ( 12 i ) = 11 10 ! 11 ! = 1 1 2 = 121 f(12)=k\prod_{i=1}^{10}(12-i)=\frac{11}{10!}11!=11^2=\boxed{121} .

Hung Woei Neoh
Jul 13, 2016

f ( x ) f(x) has roots 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 1,2,3,4,5,6,7,8,9,10 . Therefore,

f ( x ) = k ( x 1 ) ( x 2 ) ( x 3 ) ( x 4 ) ( x 5 ) ( x 6 ) ( x 7 ) ( x 8 ) ( x 9 ) ( x 10 ) f(x) = k(x-1)(x-2)(x-3)(x-4)(x-5)(x-6)(x-7)(x-8)(x-9)(x-10)

f ( 11 ) = 11 k ( 11 1 ) ( 11 2 ) ( 11 3 ) ( 11 4 ) ( 11 5 ) ( 11 6 ) ( 11 7 ) ( 11 8 ) ( 11 9 ) ( 11 10 ) = 11 k ( 10 ) ( 9 ) ( 8 ) ( 7 ) ( 6 ) ( 5 ) ( 4 ) ( 3 ) ( 2 ) ( 1 ) = 11 k = 11 10 ! f(11)=11\\ k(11-1)(11-2)(11-3)(11-4)(11-5)(11-6)(11-7)(11-8)(11-9)(11-10)=11\\ k(10)(9)(8)(7)(6)(5)(4)(3)(2)(1)=11\\ \implies k =\dfrac{11}{10!}

We then calculate f ( 12 ) f(12) :

f ( 12 ) = 11 10 ! ( 12 1 ) ( 12 2 ) ( 12 3 ) ( 12 4 ) ( 12 5 ) ( 12 6 ) ( 12 7 ) ( 12 8 ) ( 12 9 ) ( 12 10 ) = 11 10 ! ( 11 ) ( 10 ) ( 9 ) ( 8 ) ( 7 ) ( 6 ) ( 5 ) ( 4 ) ( 3 ) ( 2 ) = 1 1 2 10 ! ( 10 ! ) = 121 f(12)=\dfrac{11}{10!}(12-1)(12-2)(12-3)(12-4)(12-5)(12-6)(12-7)(12-8)(12-9)(12-10)\\ =\dfrac{11}{10!}(11)(10)(9)(8)(7)(6)(5)(4)(3)(2)\\ =\dfrac{11^2}{\color{#D61F06}{\cancel{\color{#333333}{10!}}}}(\color{#D61F06}{\cancel{\color{#333333}{10!}}})\\ =\boxed{121}

The real purpose of me writing this solution:

To tell you that

f ( x ) = 11 3628800 x 10 121 725760 x 9 + 121 30240 x 8 1331 24192 x 7 + 82643 172800 x 6 94501 34560 x 5 + 3758623 362880 x 4 925045 36288 x 3 + 1948463 50400 x 2 81191 2520 x + 11 f(x)=\dfrac{11}{3628800}x^{10}-\dfrac{121}{725760}x^9+\dfrac{121}{30240}x^8-\dfrac{1331}{24192}x^7+\dfrac{82643}{172800}x^6\\-\dfrac{94501}{34560}x^5+\dfrac{3758623}{362880}x^4-\dfrac{925045}{36288}x^3+\dfrac{1948463}{50400}x^2-\dfrac{81191}{2520}x+11

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