A Tribute to an Ingenious Mind: Kishlaya Jaiswal

Calculus Level 5

Let f ( x ) f(x) be a function with two properties:

(a) For any two real numbers x x and y y , f ( x + y ) = x + f ( y ) f(x+y) = x + f(y) and

(b) f ( 0 ) = 2 f(0)=2

If α \alpha is a Real Number and both β \beta and γ \gamma are positive integers

arcsin ( f ( 100 ) ) = α i ln ( β + γ ) \arcsin (f(100)) = \alpha - i\ln(\beta+\sqrt{\gamma})

Evaluate α β γ \lfloor \alpha \beta \gamma \rfloor .


The answer is 1666781.

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2 solutions

The solution for the first part of the question has been provided by my friend Kish below

The value of ζ = f ( 100 ) \zeta= f(100) evaluates to be 102 102 .

We all know that arcsin ( n ) = i l n ( i n ± 1 n 2 ) = i l n ( i n ± i m = i l n ( i ( n ± m ) ) \arcsin (n) = -i\cdot ln(i\cdot n \pm \sqrt{1-n^{2}}) \\= -i\cdot ln(i\cdot n \pm i\sqrt{m} \\= -i\cdot ln(i(n \pm \sqrt{m}))

Now we use e i x = c o s ( x ) + i s i n ( x ) and = l n ( i x ) = l n ( x e i π 2 ) = i ( l n ( x ) + i π 2 ) = π 2 l n ( x ) e^{ix} = cos(x) + i\cdot sin(x) \\ \quad \text{and} \quad \\= ln(i\cdot x) = ln(x\cdot e^{i\cdot \frac{\pi}{2}}) \\= -i\cdot ( ln(x) + i\cdot \frac{\pi}{2}) \\= \frac{\pi}{2} - ln(x)

So we get π 2 i l n ( n ± m ) \\ \rightarrow \frac{\pi}{2} - i\cdot ln(n \pm \sqrt{m})

So we have arcsin ( 102 ) = π 2 i l n ( 102 + 101 103 ) \arcsin (102) = \frac{\pi}{2} - i\cdot ln(102 + \sqrt{101\cdot 103}) .

Therefore the product evaluates to 1666781.407 1666781.407 and hence the final answer is 1666781 1666781 .

I would like to add to Azhaghu Roopesh M solution, on how to find the value of ζ \zeta . It follows as :

Setting y = 0 y=0 in the functional equation, we get

f ( x ) = x + f ( 0 ) = x + 2 f(x) = x + f(0) = x+2

And thus,

ζ = f ( 100 ) = 100 + 2 = 102 \zeta = f(100) = 100+2 = 102 .

The rest of the work has already been done by my dear friend.

Kishlaya Jaiswal - 6 years, 3 months ago

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Nice wiki (Y)

Krishna Sharma - 6 years, 3 months ago

I've cleaned the question up a bit. Nice problem.

Jake Lai - 6 years, 3 months ago

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Yeah, looks good now . Thanks Jake ¨ \ddot\smile

A Former Brilliant Member - 6 years, 3 months ago

Many many thanks to @Kishlaya Jaiswal for introducing me to finding complex solutions of Inverse Trigonometric Ratios .

@Calvin Lin I wanted to make this question a community post for this wiki by Kishlaya , but I couldn't do so . Can you please help me out ?

Thanks sir !!

A Former Brilliant Member - 6 years, 3 months ago

If anyone wants to know how I began with the proof , please visit this Brilliant wiki by Kishlaya Jaiswal .

A Former Brilliant Member - 6 years, 3 months ago

Okay, so let's begin by defining f ( x ) f(x) . Set y = 0 y = 0 ; this gives f ( x ) = x + f ( 0 ) = x + 2 f(x) = x + f(0) = x + 2 .

This gives us ζ = f ( 100 ) = 100 + 2 = 102 \zeta = f(100) = 100 + 2 = 102 . We want to find s i n 1 ( 102 ) sin^{-1}(102)

Now comes the tricky part. I assume that the reader is familiar with Euler's formula, which is e i θ = c o s ( θ ) + i s i n ( θ ) e^{i\theta} = cos(\theta) + isin(\theta) ; from it we can derive the following:

s i n ( θ ) = e i θ e i θ 2 sin(\theta) = \frac{e^{i\theta} - e^{-i\theta}}{2}

Let e i θ = y e^{i*\theta} = y . We have: y 1 y = 204 i y - \frac{1}{y} = 204i ; solving for y yields:

y = 204 i ± 41616 + 4 2 = 204 i ± 2 i 10403 2 y = \frac{204i \pm \sqrt{-41616 + 4}}{2} = \frac{204i \pm 2i\sqrt{10403}}{2}

y = 102 i ± i 10403 y = 102i \pm i\sqrt{10403}

Thus, i θ = l n ( y ) = l n ( i ) + l n ( 102 ± 10403 ) i\theta = ln(y) = ln (i) + ln (102 \pm \sqrt{10403}) . I assume that the final answer you seek is the principal value of the expression; in other words, l n ( i ) = i π 2 ln (i) = \frac{i\pi}{2} .

Therefore, θ = π 2 i l n ( 102 + 10403 ) \theta = \frac{\pi}{2} - i*ln(102 + \sqrt{10403}) ; the product ω \omega equals 102 10403 π 2 1666781.4 \frac{102*10403*\pi}{2} \approx 1666781.4 ; thus, the greatest integer smaller than ω \omega is 1666781 1666781 .

same method by me too

Ashutosh Sharma - 3 years, 2 months ago

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