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JEE Main 2023
Definite Integration
Definite Integration
Hard

Question

Let f(x)=min{[x1],[x2],,[x10]}f(x)=\min \{[x-1],[x-2], \ldots,[x-10]\} where [t] denotes the greatest integer t\leq \mathrm{t}. Then 010f(x)dx+010(f(x))2 dx+010f(x)dx\int\limits_{0}^{10} f(x) \mathrm{d} x+\int\limits_{0}^{10}(f(x))^{2} \mathrm{~d} x+\int\limits_{0}^{10}|f(x)| \mathrm{d} x is equal to ________________.

Answer: 1

Solution

Key Concepts and Formulas:

  • Greatest Integer Function Property: For any real number xx and integer kk, [xk]=[x]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|y| = y if y0y \ge 0, and y=y|y| = -y if y<0y < 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=N(N+1)(2N+1)6\sum_{n=1}^{N} n^2 = \frac{N(N+1)(2N+1)}{6}.

Step-by-Step Solution:

1. Simplify the function f(x)f(x)

The function is given by f(x)=min{[x1],[x2],,[x10]}f(x)=\min \{[x-1],[x-2], \ldots,[x-10]\}. We can use the property of the greatest integer function: [tk]=[t]k[t-k] = [t]-k for any integer kk. Applying this to each term within the minimum function: [x1]=[x]1[x-1] = [x]-1 [x2]=[x]2[x-2] = [x]-2 \vdots [x10]=[x]10[x-10] = [x]-10

So, f(x)=min{[x]1,[x]2,,[x]10}f(x) = \min \{[x]-1, [x]-2, \ldots, [x]-10\}. Let I=[x]I = [x]. Then f(x)=min{I1,I2,,I10}f(x) = \min \{I-1, I-2, \ldots, I-10\}. The values I1,I2,,I10I-1, I-2, \ldots, I-10 are in decreasing order. Therefore, the minimum value is I10I-10. Thus, f(x)=[x]10f(x) = [x]-10. Using the greatest integer function property in reverse, we can write f(x)=[x10]f(x) = [x-10].

2. Simplify the integral expression using the properties of f(x)f(x) and absolute value

The expression to evaluate is E=010f(x)dx+010(f(x))2 dx+010f(x)dxE = \int\limits_{0}^{10} f(x) \mathrm{d} x+\int\limits_{0}^{10}(f(x))^{2} \mathrm{~d} x+\int\limits_{0}^{10}|f(x)| \mathrm{d} x. We established that f(x)=[x10]f(x) = [x-10]. We need to determine the sign of f(x)f(x) for x[0,10]x \in [0, 10]. For x[0,10]x \in [0, 10], the term x10x-10 ranges from 010=100-10 = -10 to 1010=010-10 = 0. So, x10[10,0]x-10 \in [-10, 0]. This implies that f(x)=[x10]f(x) = [x-10] will always be a non-positive integer. Specifically, f(x)0f(x) \le 0 for all x[0,10]x \in [0, 10].

Since f(x)0f(x) \le 0 for x[0,10]x \in [0, 10], we can simplify f(x)|f(x)|. According to the definition of absolute value, if y0y \le 0, then y=y|y| = -y. Therefore, f(x)=f(x)|f(x)| = -f(x) for x[0,10]x \in [0, 10].

Substitute this into the expression for EE: E=010f(x)dx+010(f(x))2 dx+010(f(x))dxE = \int\limits_{0}^{10} f(x) \mathrm{d} x+\int\limits_{0}^{10}(f(x))^{2} \mathrm{~d} x+\int\limits_{0}^{10}(-f(x)) \mathrm{d} x Using the linearity of definite integrals, we can combine the first and third integrals: E=(010f(x)dx010f(x)dx)+010(f(x))2 dxE = \left(\int\limits_{0}^{10} f(x) \mathrm{d} x - \int\limits_{0}^{10} f(x) \mathrm{d} x\right) + \int\limits_{0}^{10}(f(x))^{2} \mathrm{~d} x The first two terms cancel out: E=010(f(x))2 dxE = \int\limits_{0}^{10}(f(x))^{2} \mathrm{~d} x

3. Evaluate the remaining integral

We need to compute E=010([x10])2 dxE = \int\limits_{0}^{10} ([x-10])^{2} \mathrm{~d} x. We know f(x)=[x10]=[x]10f(x) = [x-10] = [x]-10. The greatest integer function [x][x] is a step function, changing its value at integer points. We can evaluate the integral by breaking the interval [0,10][0, 10] into unit sub-intervals: [0,1),[1,2),,[9,10)[0, 1), [1, 2), \ldots, [9, 10).

In each sub-interval [k,k+1)[k, k+1) for k{0,1,,9}k \in \{0, 1, \ldots, 9\}, the value of [x][x] is kk. Therefore, for x[k,k+1)x \in [k, k+1), f(x)=[x]10=k10f(x) = [x]-10 = k-10.

The integral can be written as a sum of integrals over these sub-intervals: E=k=09kk+1([x]10)2dxE = \sum_{k=0}^{9} \int_{k}^{k+1} ([x]-10)^2 \mathrm{d} x Substituting the constant value of ([x]10)([x]-10) in each interval: E=01(010)2dx+12(110)2dx+23(210)2dx++910(910)2dxE = \int_{0}^{1} (0-10)^2 \mathrm{d} x + \int_{1}^{2} (1-10)^2 \mathrm{d} x + \int_{2}^{3} (2-10)^2 \mathrm{d} x + \ldots + \int_{9}^{10} (9-10)^2 \mathrm{d} x E=01(10)2dx+12(9)2dx+23(8)2dx++910(1)2dxE = \int_{0}^{1} (-10)^2 \mathrm{d} x + \int_{1}^{2} (-9)^2 \mathrm{d} x + \int_{2}^{3} (-8)^2 \mathrm{d} x + \ldots + \int_{9}^{10} (-1)^2 \mathrm{d} x For a constant CC, abC2dx=C2(ba)\int_{a}^{b} C^2 \mathrm{d} x = C^2 (b-a). Since the length of each interval is 1, each integral evaluates to the square of the constant: E=(10)2(10)+(9)2(21)+(8)2(32)++(1)2(109)E = (-10)^2 \cdot (1-0) + (-9)^2 \cdot (2-1) + (-8)^2 \cdot (3-2) + \ldots + (-1)^2 \cdot (10-9) E=(10)2+(9)2+(8)2++(1)2E = (-10)^2 + (-9)^2 + (-8)^2 + \ldots + (-1)^2 E=102+92+82++12E = 10^2 + 9^2 + 8^2 + \ldots + 1^2 This is the sum of the squares of the first 10 natural numbers. Using the sum of squares formula n=1Nn2=N(N+1)(2N+1)6\sum_{n=1}^{N} n^2 = \frac{N(N+1)(2N+1)}{6} with N=10N=10: E=10(10+1)(210+1)6E = \frac{10(10+1)(2 \cdot 10+1)}{6} E=1011216E = \frac{10 \cdot 11 \cdot 21}{6} E=23106E = \frac{2310}{6} E=385E = 385


Common Mistakes & Tips:

  • Simplifying f(x)f(x): Ensure correct application of the [xk]=[x]k[x-k] = [x]-k property. Misinterpreting the minimum of the set {[x]1,,[x]10}\{[x]-1, \ldots, [x]-10\} can lead to an incorrect f(x)f(x).
  • Absolute Value Analysis: Carefully determine the sign of f(x)f(x) over the integration interval. Incorrectly simplifying f(x)|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 [xk]=[x]k[x-k] = [x]-k, we simplified f(x)f(x) to [x10][x-10]. Over the interval [0,10][0, 10], f(x)f(x) is always non-positive, which simplified the absolute value term f(x)|f(x)| to f(x)-f(x). This led to the cancellation of the f(x)dx\int f(x) dx and f(x)dx\int -f(x) dx terms, leaving only (f(x))2dx\int (f(x))^2 dx. The integral of ([x10])2([x-10])^2 over [0,10][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\boxed{385}.

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