It's about to hit the floor

Calculus Level 5

1 1 { x 2 + x 5 } d x \large\int_{-1}^{1}\{x^2+x-5\} \, dx

If the value of the integral above is of the form 1 a ( b c d ) \frac{1}{a}(b\sqrt{c}-d) where a , b , c , d a,b,c,d are positive integers and b , c b,c are primes, find the value of a + b + c + d a+b+c+d .

Note: { x } = { x x , x 0 x x , x < 0 \{x\}=\begin{cases}{x-\lfloor{x}\rfloor \quad , \quad x\geq0} \\ { x-\lceil{x}\rceil \quad , \quad x<0} \end{cases} .


The answer is 25.

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1 solution

Adarsh Kumar
Jul 5, 2015

I would first like to thank Pi Han Goh for telling me that fractional function is different for positive and negative x! Without Pi Han i could never have posted this problem!

First we divide the integral into two parts, 1 0 { x 2 + x 5 } d x + 0 1 { x 2 + x 5 } d x \int_{-1}^{0}\{x^2+x-5\}dx+\int_{0}^{1}\{x^2+x-5\}dx .This is done as the definition of fractional function is different when x x is negative and positive.Now,we rewrite the integrals as, 1 0 { x 2 + x 5 } d x = 1 0 ( ( x 2 + x 5 ) x 2 + x 5 ) d x \int_{-1}^{0}\{x^2+x-5\}dx=\int_{-1}^{0}((x^2+x-5)-\lceil{x^2+x-5}\rceil)dx .And, 0 1 { x 2 + x 5 } d x = 0 1 ( ( x 2 + x 5 ) x 2 + x 5 ) d x \int_{0}^1\{x^2+x-5\}dx=\int_{0}^{1}((x^2+x-5)-\lfloor{x^2+x-5}\rfloor)dx .The first part of both the integrals can be calculated quite easily,hence we leave that and move on to the second part of the first integral,since 1 < x < 0 , 0 < ( x + 1 ) < 1 x ( x + 1 ) 5 = 5 -1<x<0,0<(x+1)<1\Longrightarrow \lceil{x(x+1)-5}\rceil=-5 .Now,we can easily calculate the second part of the first integral.Now,we move on to the second part of the second integral, 0 1 x 2 + x 5 d x = 0 1 ( x 2 + x 5 ) d x \int_{0}^{1}\lfloor{x^2+x-5}\rfloor dx=\int_{0}^{1}(\lfloor{x^2+x}\rfloor-5)dx .Now,we just have to calculate the first part as the second part is quite easy,so x 2 + x = 1 o r x 2 + x 1 = 0 x^2+x=1\ or\ x^2+x-1=0 has only one solution in our interval and that is 5 1 2 \dfrac{\sqrt{5}-1}{2} which implies that when x x ranges from 0 0 to this value the integral part would be 0 0 hence we just calculate it from 5 1 2 \dfrac{\sqrt{5}-1}{2} to 1 1 . 5 1 2 1 x 2 + x 1 = 5 1 2 1 1 d x \int_{\frac{\sqrt{5}-1}{2}}^{1}\lfloor{x^2+x-1}\rfloor=\int_{\frac{\sqrt{5}-1}{2}}^{1}{1dx} .Now it is just a matter of calculations before we reach our answer.And done!

Moderator note:

Great. The tricky part of this problem is finding the points where the function becomes discontinuous, and then figuring out what to do with each section.

Note: I prefer the other definition of floor function, which would allow us to remove the -5 term completely.

@Pi Han Goh is this solution correct now?

Adarsh Kumar - 5 years, 11 months ago

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Haha, yeah! It would be better to fill in the calculations (like in the report section).

By the way, your old problem is right too if you allow the definition of { x } = x x \{x\} = x - \lfloor x \rfloor for all real x x .

Pi Han Goh - 5 years, 11 months ago

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There are actually conflicting definitions for what the fractional part of a negative number is.

To me, { x } = x x \{ x \} = x - \lfloor x \rfloor . I'm glad that you included the definition here.

Note: I am not a fan of this definition, because it ends up with x 1 x \lfloor x - 1 \rfloor \neq \lfloor x \rfloor for 0 < x < 1 0 < x < 1 .

Calvin Lin Staff - 5 years, 11 months ago

{x²+x-1} has a floor definition when "x²+x-1" <0. Not x.

{x²+x-1} has a ceiling definition when "x²+x-1" >=0. Not x.

So you split the integral at x=0 while "x²+x-1" is always negative (strictly) on whole interval [-1;1].

Leonblum Iznotded - 2 years, 10 months ago

In all of the books(Indian books) I have studied about real analysis or calculus...I have never found the definition of fractional part of x to be x- ceil(x) for negative x. I have always seen the definition to be x-floor(x) for all real x. Since in all cases the set of points of discontinuity is a set of measure zero we can say that it is Riemann integrable and then proceed. The answer if you consider the floor definition comes out to be 3 5 + 1 6 \frac{3\sqrt{5}+1}{6} And in the ceil definition used in this question...the answer would be. 3 5 11 6 \frac{3\sqrt{5}-11}{6} . Frankly I don't see the point of using this weird definition of fractional part as brilliant wiki also use the floor definition.
Anyways...the difference between these to integral is just by a factor of 2. So we can add or substrsct it according to the definition which is being used.

Arghyadeep Chatterjee - 4 months, 1 week ago

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