Let's limit the flexible polynomial

Calculus Level 5

Consider P ( x ) = 1729 x n + a n 1 x n 1 + + a 1 x + a 0 P(x)=1729x^n+a_{n-1}x^{n-1}+\dots+a_1x+a_0 , a i R \forall \ a_i \in \mathbb{R} with 0 i n 1 0\leq i \leq n-1 .

Let α 1 , α 2 , , α n \alpha_1,\alpha_2,\dots ,\alpha_n be its n n roots such that i = 2 n ( α 1 α i ) = k \displaystyle\prod_{i=2}^{n} (\alpha_1-\alpha_i) =k where k k is some real number other than 0 0 .

If L = lim x α 1 ( 1 + P ( x ) ) 1 / ( x α 1 ) L=\displaystyle\lim_{x\rightarrow \alpha_1} \left(1+P(x)\right)^{1/(x-\alpha_1)} where α 1 \alpha_1 is a real root, then evaluate ln L k \dfrac{\ln L}{k} to correct two decimal places.


This is a generalized form of one of my homework problems.


The answer is 1729.00.

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1 solution

Nihar Mahajan
Dec 18, 2016

We can express P ( x ) = 1729 i = 1 n ( x α i ) P(x)=1729\displaystyle\prod_{i=1}^{n}(x-\alpha_i) and we can write L L as lim x > α 1 ( 1 + 1729 i = 1 n ( x α i ) ) 1729 i = 2 n ( x α i ) 1729 i = 1 n ( x α i ) \displaystyle\lim_{x->\alpha_1}\left(1+1729\displaystyle\prod_{i=1}^{n}(x-\alpha_i)\right)^{\frac{1729\displaystyle\prod_{i=2}^{n}(x-\alpha_i)}{1729\displaystyle\prod_{i=1}^{n}(x-\alpha_i)}} . Note that as x α 1 x\rightarrow \alpha_1 , 1729 i = 1 n ( x α i ) 0 1729\displaystyle\prod_{i=1}^{n}(x-\alpha_i) \rightarrow 0 , so the limit L L is equivalent to the famous limit lim x 0 ( 1 + x ) 1 / x = e \displaystyle\lim_{x\rightarrow 0} (1+x)^{1/x} = e . So finally, we have:

L = e lim x α 1 1729 i = 2 n ( x α i ) = e 1729 k L=e^{\displaystyle\lim_{x\rightarrow \alpha_1} 1729\displaystyle\prod_{i=2}^{n}(x-\alpha_i)}=e^{1729k} and we have ln L k = 1729 \dfrac{\ln L}{k}=\boxed{1729} .

Damn! I forgot the constant 1729 while writing P(x).

BTW nice problem.

Harsh Shrivastava - 4 years, 6 months ago

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Lol, even I had forgot about the constant when I first attempted this problem :P

Nihar Mahajan - 4 years, 5 months ago

well yeah!i was wondering why its not 1 till i realised i forgot the constant 1729!

Rohith M.Athreya - 4 years, 5 months ago

Why you gave answer as decimals??

shivamani patil - 4 years, 5 months ago

I think my answer 1729 was not accepted as it show ans 1729.00

Dhruv Joshi - 4 years, 2 months ago

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