Key Concepts and Formulas:
- Greatest Integer Function Property: For any real number x and integer k, [x−k]=[x]−k.
- Definition of Minimum: The minimum of a set of numbers is the smallest value in that set.
- Absolute Value Definition: ∣y∣=y if y≥0, and ∣y∣=−y if y<0.
- Definite Integral of a Step Function: If a function is piecewise constant over an interval, its definite integral is the sum of the products of the constant value and the length of the sub-interval.
- Sum of Squares Formula: ∑n=1Nn2=6N(N+1)(2N+1).
Step-by-Step Solution:
1. Simplify the function f(x)
The function is given by f(x)=min{[x−1],[x−2],…,[x−10]}.
We can use the property of the greatest integer function: [t−k]=[t]−k for any integer k.
Applying this to each term within the minimum function:
[x−1]=[x]−1
[x−2]=[x]−2
⋮
[x−10]=[x]−10
So, f(x)=min{[x]−1,[x]−2,…,[x]−10}.
Let I=[x]. Then f(x)=min{I−1,I−2,…,I−10}.
The values I−1,I−2,…,I−10 are in decreasing order. Therefore, the minimum value is I−10.
Thus, f(x)=[x]−10.
Using the greatest integer function property in reverse, we can write f(x)=[x−10].
2. Simplify the integral expression using the properties of f(x) and absolute value
The expression to evaluate is E=0∫10f(x)dx+0∫10(f(x))2 dx+0∫10∣f(x)∣dx.
We established that f(x)=[x−10]. We need to determine the sign of f(x) for x∈[0,10].
For x∈[0,10], the term x−10 ranges from 0−10=−10 to 10−10=0.
So, x−10∈[−10,0].
This implies that f(x)=[x−10] will always be a non-positive integer. Specifically, f(x)≤0 for all x∈[0,10].
Since f(x)≤0 for x∈[0,10], we can simplify ∣f(x)∣. According to the definition of absolute value, if y≤0, then ∣y∣=−y.
Therefore, ∣f(x)∣=−f(x) for x∈[0,10].
Substitute this into the expression for E:
E=0∫10f(x)dx+0∫10(f(x))2 dx+0∫10(−f(x))dx
Using the linearity of definite integrals, we can combine the first and third integrals:
E=(0∫10f(x)dx−0∫10f(x)dx)+0∫10(f(x))2 dx
The first two terms cancel out:
E=0∫10(f(x))2 dx
3. Evaluate the remaining integral
We need to compute E=0∫10([x−10])2 dx.
We know f(x)=[x−10]=[x]−10. The greatest integer function [x] is a step function, changing its value at integer points. We can evaluate the integral by breaking the interval [0,10] into unit sub-intervals: [0,1),[1,2),…,[9,10).
In each sub-interval [k,k+1) for k∈{0,1,…,9}, the value of [x] is k.
Therefore, for x∈[k,k+1), f(x)=[x]−10=k−10.
The integral can be written as a sum of integrals over these sub-intervals:
E=∑k=09∫kk+1([x]−10)2dx
Substituting the constant value of ([x]−10) in each interval:
E=∫01(0−10)2dx+∫12(1−10)2dx+∫23(2−10)2dx+…+∫910(9−10)2dx
E=∫01(−10)2dx+∫12(−9)2dx+∫23(−8)2dx+…+∫910(−1)2dx
For a constant C, ∫abC2dx=C2(b−a). Since the length of each interval is 1, each integral evaluates to the square of the constant:
E=(−10)2⋅(1−0)+(−9)2⋅(2−1)+(−8)2⋅(3−2)+…+(−1)2⋅(10−9)
E=(−10)2+(−9)2+(−8)2+…+(−1)2
E=102+92+82+…+12
This is the sum of the squares of the first 10 natural numbers. Using the sum of squares formula ∑n=1Nn2=6N(N+1)(2N+1) with N=10:
E=610(10+1)(2⋅10+1)
E=610⋅11⋅21
E=62310
E=385
Common Mistakes & Tips:
- Simplifying f(x): Ensure correct application of the [x−k]=[x]−k property. Misinterpreting the minimum of the set {[x]−1,…,[x]−10} can lead to an incorrect f(x).
- Absolute Value Analysis: Carefully determine the sign of f(x) over the integration interval. Incorrectly simplifying ∣f(x)∣ will alter the integral.
- Integral of Step Functions: Break the integral at integer points where the greatest integer function changes. The integral of a constant over an interval is that constant multiplied by the interval's length.
Summary:
The problem involves simplifying a function defined using the minimum of greatest integer functions, and then evaluating a definite integral expression. By using the property [x−k]=[x]−k, we simplified f(x) to [x−10]. Over the interval [0,10], f(x) is always non-positive, which simplified the absolute value term ∣f(x)∣ to −f(x). This led to the cancellation of the ∫f(x)dx and ∫−f(x)dx terms, leaving only ∫(f(x))2dx. The integral of ([x−10])2 over [0,10] was then evaluated by splitting the integral into unit intervals, resulting in the sum of squares of integers from 1 to 10, which was calculated using the standard formula.
The final answer is 385.