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Definite Integration question

2010 · Shift 1 · Q48
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Definite Integration question

2010 · Shift 1 · Q48

JEE AdvancedMathematicsDefinite IntegrationMCQ+4 / −1
The value of ∫01x4(1−x)41+x2dx\int\limits_0^1 {{{{x^4}{{\left( {1 - x} \right)}^4}} \over {1 + {x^2}}}dx}0∫1​1+x2x4(1−x)4​dx is (are)
  1. A
    227−π{{22} \over 7} - \pi722​−π
  2. B
    2105{2 \over {105}}1052​
  3. C
    000
  4. D
    7115−3π2{{71} \over {15}} - {{3\pi } \over 2}1571​−23π​
View written solutionFree

Correct answer: A

The user wants me to solve the definite integral: I=∫01x4(1−x)41+x2dxI = \int\limits_0^1 {{{{x^4}{{\left( {1 - x} \right)}^4}} \over {1 + {x^2}}}dx}I=0∫1​1+x2x4(1−x)4​dx

This is a well-known integral related to an approximation of π\piπ. Let's solve it from first principles.

Step 1: Expand the numerator. The numerator is N(x)=x4(1−x)4=(x(1−x))4=(x−x2)4N(x) = x^4(1-x)^4 = (x(1-x))^4 = (x-x^2)^4N(x)=x4(1−x)4=(x(1−x))4=(x−x2)4. Using the binomial expansion for (a−b)4=a4−4a3b+6a2b2−4ab3+b4(a-b)^4 = a^4 - 4a^3b + 6a^2b^2 - 4ab^3 + b^4(a−b)4=a4−4a3b+6a2b2−4ab3+b4. Here, a=xa=xa=x and b=x2b=x^2b=x2. So, N(x)=(x)4−4(x)3(x2)+6(x)2(x2)2−4(x)(x2)3+(x2)4N(x) = (x)^4 - 4(x)^3(x^2) + 6(x)^2(x^2)^2 - 4(x)(x^2)^3 + (x^2)^4N(x)=(x)4−4(x)3(x2)+6(x)2(x2)2−4(x)(x2)3+(x2)4 N(x)=x4−4x5+6x6−4x7+x8N(x) = x^4 - 4x^5 + 6x^6 - 4x^7 + x^8N(x)=x4−4x5+6x6−4x7+x8.

So the integral becomes: I=∫01x8−4x7+6x6−4x5+x41+x2dxI = \int_0^1 \frac{x^8 - 4x^7 + 6x^6 - 4x^5 + x^4}{1+x^2} dxI=∫01​1+x2x8−4x7+6x6−4x5+x4​dx

Step 2: Perform polynomial long division. We need to divide the numerator polynomial P(x)=x8−4x7+6x6−4x5+x4P(x) = x^8 - 4x^7 + 6x^6 - 4x^5 + x^4P(x)=x8−4x7+6x6−4x5+x4 by the denominator polynomial D(x)=x2+1D(x) = x^2 + 1D(x)=x2+1.

Performing the long division:

        x^6 - 4x^5 + 5x^4 - 4x^2 + 4      _________________________________x^2+1 | x^8 - 4x^7 + 6x^6 - 4x^5 + x^4       -(x^8         + x^6)      _________________________________            -4x^7 + 5x^6 - 4x^5           -(-4x^7         - 4x^5)           _________________________                   5x^6 + 0x^5 + x^4                  -(5x^6         + 5x^4)                  _________________________                          -4x^4                         -(-4x^4         - 4x^2)                         _________________________                                  4x^2                                 -(4x^2 + 4)                                 ____________                                        -4

So, the division gives a quotient Q(x)=x6−4x5+5x4−4x2+4Q(x) = x^6 - 4x^5 + 5x^4 - 4x^2 + 4Q(x)=x6−4x5+5x4−4x2+4 and a remainder R(x)=−4R(x) = -4R(x)=−4. Therefore, the integrand can be written as: x8−4x7+6x6−4x5+x41+x2=(x6−4x5+5x4−4x2+4)−41+x2\frac{x^8 - 4x^7 + 6x^6 - 4x^5 + x^4}{1+x^2} = (x^6 - 4x^5 + 5x^4 - 4x^2 + 4) - \frac{4}{1+x^2}1+x2x8−4x7+6x6−4x5+x4​=(x6−4x5+5x4−4x2+4)−1+x24​

Step 3: Integrate the resulting expression. The integral now is: I=∫01(x6−4x5+5x4−4x2+4−41+x2)dxI = \int_0^1 \left( x^6 - 4x^5 + 5x^4 - 4x^2 + 4 - \frac{4}{1+x^2} \right) dxI=∫01​(x6−4x5+5x4−4x2+4−1+x24​)dx

Let's integrate term by term: I=[x77−4x66+5x55−4x33+4x−4tan⁡−1(x)]01I = \left[ \frac{x^7}{7} - 4\frac{x^6}{6} + 5\frac{x^5}{5} - 4\frac{x^3}{3} + 4x - 4 \tan^{-1}(x) \right]_0^1I=[7x7​−46x6​+55x5​−43x3​+4x−4tan−1(x)]01​ I=[x77−2x63+x5−4x33+4x−4tan⁡−1(x)]01I = \left[ \frac{x^7}{7} - \frac{2x^6}{3} + x^5 - \frac{4x^3}{3} + 4x - 4 \tan^{-1}(x) \right]_0^1I=[7x7​−32x6​+x5−34x3​+4x−4tan−1(x)]01​

Step 4: Evaluate the definite integral. Now, substitute the limits of integration. The value at the lower limit x=0x=0x=0 is 0 for all terms. So, we just need to evaluate at the upper limit x=1x=1x=1.

I=(177−2(1)63+15−4(1)33+4(1)−4tan⁡−1(1))−(0)I = \left( \frac{1^7}{7} - \frac{2(1)^6}{3} + 1^5 - \frac{4(1)^3}{3} + 4(1) - 4 \tan^{-1}(1) \right) - (0)I=(717​−32(1)6​+15−34(1)3​+4(1)−4tan−1(1))−(0) I=17−23+1−43+4−4(π4)I = \frac{1}{7} - \frac{2}{3} + 1 - \frac{4}{3} + 4 - 4 \left(\frac{\pi}{4}\right)I=71​−32​+1−34​+4−4(4π​) I=17−(23+43)+(1+4)−πI = \frac{1}{7} - \left(\frac{2}{3} + \frac{4}{3}\right) + (1+4) - \piI=71​−(32​+34​)+(1+4)−π I=17−63+5−πI = \frac{1}{7} - \frac{6}{3} + 5 - \piI=71​−36​+5−π I=17−2+5−πI = \frac{1}{7} - 2 + 5 - \piI=71​−2+5−π I=17+3−πI = \frac{1}{7} + 3 - \piI=71​+3−π

Now, combine the fractions: I=1+3×77−πI = \frac{1 + 3 \times 7}{7} - \piI=71+3×7​−π I=1+217−πI = \frac{1 + 21}{7} - \piI=71+21​−π I=227−πI = \frac{22}{7} - \piI=722​−π

Step 5: Compare with the options. The calculated value is I=227−πI = \frac{22}{7} - \piI=722​−π. Let's check the given options: A: 227−π\frac{22}{7} - \pi722​−π B: 2105\frac{2}{105}1052​ C: 000 D: 7115−3π2\frac{71}{15} - \frac{3\pi}{2}1571​−23π​

Our result matches option A exactly.

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        x^6 - 4x^5 + 5x^4 - 4x^2 + 4      _________________________________x^2+1 | x^8 - 4x^7 + 6x^6 - 4x^5 + x^4       -(x^8         + x^6)      _________________________________            -4x^7 + 5x^6 - 4x^5           -(-4x^7         - 4x^5)           _________________________                   5x^6 + 0x^5 + x^4                  -(5x^6         + 5x^4)                  _________________________                          -4x^4                         -(-4x^4         - 4x^2)                         _________________________                                  4x^2                                 -(4x^2 + 4)                                 ____________                                        -4