He is your Opposite, your Negative.

Algebra Level 5

If a n = n 101 a_n = \frac {n}{101} , what is the value of the summation below?

n = 1 101 a n 3 1 3 a n + 3 a n 2 \large \displaystyle \sum_{n=1}^{101} \frac {a_n ^3}{1 - 3a_n + 3a_n ^2}


The answer is 51.

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1 solution

First notice, a 101 n = 101 n 101 = 1 n 101 = 1 a n a_{101-n} = \frac{101-n}{101} = 1 - \frac{n}{101} = 1 - a_n

a n = 1 a 101 n \Rightarrow a_n = 1 -a_{101-n}

a n 3 1 3 a n + 3 a n 2 + a 101 n 3 1 3 a 101 n + 3 a 101 n 2 \frac{a_n^3}{1-3a_n+3a_n^2} + \frac{a_{101-n}^3}{1-3a_{101-n}+3a_{101-n}^2}

= a n 3 ( 1 a n ) 3 + a n 3 + a 101 n 3 ( 1 a 101 n ) 3 + a 101 n 3 = \frac{a_n^3}{(1-a_n)^3+a_n^3} + \frac{a_{101-n}^3}{(1-a_{101-n})^3+a_{101-n}^3}

= a n 3 ( 1 a n ) 3 + a n 3 + ( 1 a n ) 3 a n 3 + ( 1 a n ) 3 = \frac{a_n^3}{(1-a_n)^3+a_n^3} + \frac{(1 - a_n)^3}{a_n^3+(1-a_n)^3}

= 1 = 1

Here, the summation of n n th and ( 101 n ) (101-n) th terms are 1 1 . Thus the sum from n = 1 n=1 to 100 100 is 50 50 .

The 101 101 th term is a 101 1 3 a 101 + 3 a 101 2 = 1 1 3 + 3 = 1 \frac{a_{101}}{1-3a_{101}+3a_{101}^2} = \frac{1}{1-3+3} = 1 .

So, total sum = 51 \boxed{51}

I can't believe I used a whole day to solve this instead of solving elegantly like you TT

敬全 钟 - 6 years, 10 months ago

Same technique here.

Arif Ahmed - 6 years, 9 months ago

Almost same

Figel Ilham - 6 years, 3 months ago

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