∫ − 1 1 { x 2 + x − 5 } d x
If the value of the integral above is of the form a 1 ( b c − d ) where a , b , c , d are positive integers and b , c are primes, find the value of a + b + c + d .
Note: { x } = { x − ⌊ x ⌋ , x ≥ 0 x − ⌈ x ⌉ , x < 0 .
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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?
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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 ⌋ for all real x .
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There are actually conflicting definitions for what the fractional part of a negative number is.
To me, { x } = x − ⌊ x ⌋ . 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 ⌋ for 0 < x < 1 .
{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].
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
6
3
5
+
1
And in the ceil definition used in this question...the answer would be.
6
3
5
−
1
1
.
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.
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First we divide the integral into two parts, ∫ − 1 0 { x 2 + x − 5 } d x + ∫ 0 1 { x 2 + x − 5 } d x .This is done as the definition of fractional function is different when 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 .And, ∫ 0 1 { x 2 + x − 5 } d x = ∫ 0 1 ( ( x 2 + x − 5 ) − ⌊ x 2 + x − 5 ⌋ ) d x .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 .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 .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 has only one solution in our interval and that is 2 5 − 1 which implies that when x ranges from 0 to this value the integral part would be 0 hence we just calculate it from 2 5 − 1 to 1 . ∫ 2 5 − 1 1 ⌊ x 2 + x − 1 ⌋ = ∫ 2 5 − 1 1 1 d x .Now it is just a matter of calculations before we reach our answer.And done!