Polynomial equation.

Algebra Level 5

Let p ( x ) p(x) be a polynomial that satisfies p ( 1 ) = 210 p(1)=210 and ( x + 10 ) p ( 2 x ) = ( 8 x 32 ) p ( x + 6 ) (x+10)p(2x)=(8x-32)p(x+6) for any real number x . x. Find the sum of all possible values of p ( 10 ) . p(10).


The answer is 336.

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1 solution

Haroun Meghaichi
Jul 25, 2014

First we find the degree of p p , assume that the degree is n n so we can write : p ( x ) = k = 0 n a k x k a n 0 , k { 0 , 1 , n } , a k R p(x)= \sum_{k=0}^{n}a_k x^k\ \ \ \ \ \ a_n\neq 0,\ \ \forall k\in\{0,1\dots,n\}, a_k\in \mathbb{R} Then the leading coefficient in ( x + 10 ) p ( 2 x ) (x+10)p(2x) is 2 n a n 2^n a_n . And the leading coefficient in ( 8 x 32 ) p ( x + 6 ) (8x-32)p(x+6) is 8 a n 8 a_n , then : 8 a n = 2 n a n a n 0 2 n = 8 n = 3. 8a_n=2^n a_n \overset{a_n\neq 0}{\Longleftrightarrow} 2^n =8 \Longleftrightarrow n=3. So p p at most three real roots : set x = 4 x=4 to get ( 4 + 10 ) p ( 8 ) = 0 p ( 8 ) = 0 (4+10)p(8)=0\Longrightarrow p(8)=0 set x = 10 x=-10 to get 0 = ( 80 32 ) p ( 4 ) p ( 4 ) = 0 0=(-80-32)p(-4)\Longrightarrow p(-4)=0 set x = 2 x=-2 to get : ( 2 + 10 ) p ( 4 ) = ( 16 32 ) p ( 4 ) p ( 4 ) = 0 (-2+10)p(-4)=(-16-32)p(4) \Longrightarrow p(4)=0 Then : p ( x ) = c ( x 4 ) ( x + 4 ) ( x 8 ) p(x)=c(x-4)(x+4)(x-8) , and p ( 1 ) = 210 = 105 c p(1)=210=105c then c = 2 c=2 , therefore p ( x ) = 2 ( x 4 ) ( x + 4 ) ( x 8 ) p(x)=2(x-4)(x+4)(x-8) Easily : p ( 10 ) = 2 6 14 2 = 336 p(10)= 2\cdot 6 \cdot 14 \cdot 2= \boxed{336}

Nice solution

Kïñshük Sïñgh - 6 years, 10 months ago

For any real number x means an identity of L.H.S. = R.H.S.

p(x) = 2 x^3 - 16 x^2 - 32 x + 256 is the only solution to the identity.

p(10) = 336 is the only possible value. Therefore, the sum of all possible values of p(10) is 336.

I actually solved this by taking (1, 210), (6.5, -78.75), (8, 0) and (10.625, 508.67578125) for cubic coefficients from p(x/ 2 + 6) = p(x)(x + 20)/(8 (x - 8)), p(2 x - 12) = p(x) 8 (x - 10) / (x + 4) and p(1) = 210, while plotting shows a cubic curve. p(12) = 1024 to positive extreme and negative infinity to negative extreme. All discrete points generated match with the cubic equation. Therefore the cubic equation is obtained. Checked that (0, 256) and (11, 630) are all right.

Your solution is more reasonable. However, Cramer's way is general for all polynomials.

Lu Chee Ket - 6 years, 9 months ago

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