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Calculus Level 3

0 10 ( 4 x x 2 + 16 ) m d x \large \displaystyle \int_0^{10} \left \lfloor \left ( \frac{4|x|}{x^2+16} \right)^m \right \rfloor \, dx

Given that 1 lies between the roots (of y y ) of the equation y 2 m y + 1 = 0 y^2-my + 1 = 0 , evaluate the integral above.


The answer is 0.000.

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3 solutions

Nishant Rai
Jun 3, 2015

Let f ( y ) = y 2 m y + 1 f(y) = y^2 - my +1

f ( 1 ) < 0 m > 2 f(1)<0 \Rightarrow m>2

Let z = 4 x x 2 + 16 = 4 t t 2 + 16 . . . . ( t = x ) z= \frac{4|x|}{x^2 + 16} = \frac{4t}{t^2 + 16} ....(t=|x|)

z t 2 4 t + 16 z = 0 \Rightarrow zt^2 -4t +16z =0

Since ' t t ' is real, D 0 D \geq 0

0 z 0.5 \Rightarrow 0 \leq z \leq 0.5 as z 0 z \geq 0

0 z m 1 2 m [ z m ] = 0 0 \leq z^m \leq \frac{1}{2^m} \Rightarrow [z^m]=0

Prakhar Bindal
Sep 29, 2016

Firstly m>2 .

Next as we are working in positive domain remove mod sign .

Next denominator is x+16/x which has a minimum value of 16 by AM-GM.

So The thing whose power is greater than 2 is less than 1 . hence greatest integer will give 0.

As simple as that!

f ( y ) = y 2 m y + 1 f(y) = y^2 - my + 1

Since the given parabola is opening upward and 1 lies between the roots of the equation f ( y ) = 0 f(y) = 0 ,

f ( 1 ) < 0 1 m + 1 < 0 m > 2 f(1) < 0 \Rightarrow 1 - m + 1 < 0 \Rightarrow \boxed{m>2}

The integral has non-negative limits, hence we could use A.M - G.M .

x 2 + 16 2 x 2 ( 16 ) 4 x x 2 + 16 1 2 \dfrac{x^2 + 16}{2} \geq \sqrt{x^2(16)} \Rightarrow \dfrac{4|x|}{x^2 +16} \leq \dfrac{1}{2}

( 4 x x 2 + 16 ) m = 0......... a m < 1 when a < 1 & m > 0 \lfloor (\frac{4|x|}{x^2 + 16})^m \rfloor = 0 . . . . . . . . . \because a^m < 1 \text{ when } a < 1 \text{ \& } m > 0

So the integrand is zero.

Rest is trivial.

Moderator note:

Note that since we are only integrating over the non-negative values, we could have simply dealt with x x in place of x |x| .

A slightly more interesting version of this problem would be to ask for 10 10 ( 4 x x 2 + 16 ) 2 d x \int_{-10} ^ {10} \lfloor \left( \frac{4x}{x^2+16} \right) ^2 \rfloor dx .

@Challenge Master: The function ( 4 x x 2 + 16 ) 2 \lfloor \left( \dfrac{4x}{x^2 + 16} \right )^2 \rfloor is an even function. Hence the integral would reduce to:

I = 2 0 10 ( 4 x x 2 + 16 ) 2 d x = 0 I = \displaystyle 2\int_{0}^{10} \lfloor \left (\dfrac{4x}{x^2 + 16} \right)^2 \rfloor dx = 0 since we are again evaluating this over non-negative x x .

Vishwak Srinivasan - 6 years ago

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