Will you differentiate till you reach 60!

Algebra Level 4

p ( x ) = n = 1 100 n x n + 1 \large p(x) = \sum_{n=1}^{100} {n x^n} + 1

Let p ( x ) p(x) be a polynomial of degree 100 as defined above. Find the sum of the roots of its 6 0 th 60^\text{th} derivative.


Hint: Check out this note .

99 250 \dfrac{-99}{250} 297 5 \dfrac{-297}{5} 198 5 \dfrac{-198}{5} 297 500 \dfrac{-297}{500}

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1 solution

Sabhrant Sachan
Sep 30, 2016

p ( x ) = n n = 1 100 x n + 1 p 1 ( x ) = n n = 1 100 n x n 1 p 2 ( x ) = n n = 1 100 n ( n 1 ) x n 2 From the pattern , the 60th derivative will be p 60 ( x ) = n n = 1 100 n ( n 1 ) ( n 59 ) x n 60 Sum of All roots = coefficient of x 39 coefficient of x 40 S = 99 × 99 × 98 × 40 100 × 100 × 99 × 98 × 41 = 99 250 p(x)=\displaystyle n\sum_{n=1}^{100} x^{n} +1 \\ p^{1}(x)=\displaystyle n\sum_{n=1}^{100} nx^{n-1} \\ p^{2}(x)=\displaystyle n\sum_{n=1}^{100} n(n-1)x^{n-2} \\ \text{From the pattern , the 60th derivative will be } \\ p^{60}(x)=\displaystyle n\sum_{n=1}^{100} n(n-1)\cdots (n-59)x^{n-60} \\ \text{Sum of All roots } = -\dfrac{\text{coefficient of } x^{39}}{\text{coefficient of } x^{40}} \\ S= - \dfrac{99\times\cancel{99}\times\cancel{98}\cdots \times 40}{100\times 100\times \cancel{99}\times \cancel{98} \cdots \times \cancel{41} } = \boxed{-\dfrac{99}{250}}

Good solution!

Tapas Mazumdar - 4 years, 8 months ago

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thank you :)

Sabhrant Sachan - 4 years, 8 months ago

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Wouldn't have been easier if you followed my 'mean value approach'. You wouldn't even have to look at the pattern.

Tapas Mazumdar - 4 years, 8 months ago

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