Isn't 100000 100000 such a huge number (Part 4) ?

Algebra Level 5

Let there be a function f ( x ) f(x) satisfying the relation:

f ( x ) = 10000 0 x 10000 0 x + 100 10 . f(x)=\dfrac{100000^{x}}{100000^{x}+100\sqrt{10}}.

Find 1 + k = 1 99998 f ( k 99999 ) . 1 + \displaystyle\sum_{k=1}^{99998}f(\dfrac{k}{99999}).


The answer is 50000.

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1 solution

Yash Singhal
Dec 14, 2014

Given f ( x ) = 10000 0 x 10000 0 x + 100 10 f(x)=\dfrac{100000^{x}}{100000^{x}+100\sqrt{10}} .

Now, f ( 1 x ) = 10000 0 1 x 10000 0 1 x + 100 10 f(1-x)=\dfrac{100000^{1-x}}{100000^{1-x}+100\sqrt{10}} which is equal to 100 10 10000 0 x + 100 10 \dfrac{100\sqrt{10}}{100000^{x}+100\sqrt{10}} .

Now, on keen and vigil observation, we see that:

f ( x ) + f ( 1 x ) = 1 f(x)+f(1-x)=1 .

So, f ( 1 99999 ) + f ( 99998 99999 ) = 1 f(\dfrac{1}{99999})+f(\dfrac{99998}{99999})=1

f ( 2 99999 ) + f ( 99997 99999 ) = 1 f(\dfrac{2}{99999})+f(\dfrac{99997}{99999})=1

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and so there would be 49999 49999 such pairs of sum whose each sum is 1 1 .

So, our answer is 49999 × 1 = 49999 49999\times 1=49999 .

Adding 1 1 to it, our final answer becomes 50000 \huge {50000} which is definitely huge!

Instead of adding 1, I think it would be nicer to express it as the sum from 0 to 99999. If you agree, can you update the question?

Calvin Lin Staff - 6 years, 5 months ago

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