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Binomial Theorem question

2022 · 28 Jun · Shift 2 · Q25
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  5. /2022 · 28 Jun · Shift 2 · Q25

Binomial Theorem question

2022 · 28 Jun · Shift 2 · Q25

JEE MainMathematicsBinomial TheoremMCQ+4 / −1
The term independent of x in the expansion of (1−x2+3x3)(52x3−15x2)11, xe0(1 - {x^2} + 3{x^3}){\left( {{5 \over 2}{x^3} - {1 \over {5{x^2}}}} \right)^{11}},\,x e 0(1−x2+3x3)(25​x3−5x21​)11,xe0 is :
  1. A
    740{7 \over {40}}407​
  2. B
    33200{33 \over {200}}20033​
  3. C
    39200{39 \over {200}}20039​
  4. D
    1150{11 \over {50}}5011​
View written solutionFree

Correct answer: B

  1. Let T=(1−x2+3x3)(52x3−15x2)11.T=(1-x^2+3x^3)\left(\frac{5}{2}x^3-\frac{1}{5x^2}\right)^{11}.T=(1−x2+3x3)(25​x3−5x21​)11. We need the term independent of xxx, i.e. the constant term.

  2. General term in the binomial expansion:

For (52x3−15x2)11,\left(\frac{5}{2}x^3-\frac{1}{5x^2}\right)^{11},(25​x3−5x21​)11, the general term is Tr+1=(11r)(52x3)11−r(−15x2)r,T_{r+1}=\binom{11}{r}\left(\frac{5}{2}x^3\right)^{11-r}\left(-\frac{1}{5x^2}\right)^r,Tr+1​=(r11​)(25​x3)11−r(−5x21​)r, where r=0,1,2,…,11r=0,1,2,\dots,11r=0,1,2,…,11.

  1. Simplify the power of xxx in the general term:

x3(11−r)⋅x−2r=x33−3r−2r=x33−5r.x^{3(11-r)}\cdot x^{-2r}=x^{33-3r-2r}=x^{33-5r}.x3(11−r)⋅x−2r=x33−3r−2r=x33−5r. So the general term is Tr+1=(11r)(52)11−r(−15)rx33−5r.T_{r+1}=\binom{11}{r}\left(\frac{5}{2}\right)^{11-r}\left(-\frac{1}{5}\right)^r x^{33-5r}.Tr+1​=(r11​)(25​)11−r(−51​)rx33−5r.

  1. Since the whole expression is multiplied by (1−x2+3x3)(1-x^2+3x^3)(1−x2+3x3), the constant term can arise in three ways:
  • 1×1 \times1× term with x0x^0x0 from the binomial part,
  • (−x2)×(-x^2) \times(−x2)× term with x−2x^{-2}x−2 from the binomial part,
  • (3x3)×(3x^3) \times(3x3)× term with x−3x^{-3}x−3 from the binomial part.

So we solve:

(i) For x0x^0x0:

33−5r=0  ⟹  r=33533-5r=0 \implies r=\frac{33}{5}33−5r=0⟹r=533​ Not an integer, so no such term.

(ii) For x−2x^{-2}x−2:

33−5r=−2  ⟹  5r=35  ⟹  r=7.33-5r=-2 \implies 5r=35 \implies r=7.33−5r=−2⟹5r=35⟹r=7. So we need the coefficient of x−2x^{-2}x−2 in the binomial part: (117)(52)4(−15)7.\binom{11}{7}\left(\frac{5}{2}\right)^4\left(-\frac{1}{5}\right)^7.(711​)(25​)4(−51​)7. Now, (117)=330,\binom{11}{7}=330,(711​)=330, (52)4=62516,\left(\frac{5}{2}\right)^4=\frac{625}{16},(25​)4=16625​, (−15)7=−178125.\left(-\frac{1}{5}\right)^7=-\frac{1}{78125}.(−51​)7=−781251​. Thus coefficient =330⋅62516⋅(−178125).=330\cdot \frac{625}{16}\cdot\left(-\frac{1}{78125}\right).=330⋅16625​⋅(−781251​). Since 78125=625⋅12578125=625\cdot 12578125=625⋅125, 62578125=1125.\frac{625}{78125}=\frac{1}{125}.78125625​=1251​. Hence =330⋅(−116⋅125)=330⋅(−12000)=−33200.=330\cdot\left(-\frac{1}{16\cdot 125}\right)=330\cdot\left(-\frac{1}{2000}\right)=-\frac{33}{200}.=330⋅(−16⋅1251​)=330⋅(−20001​)=−20033​.

Multiplying by −x2-x^2−x2, contribution to constant term is (−1)(−33200)=33200.(-1)\left(-\frac{33}{200}\right)=\frac{33}{200}.(−1)(−20033​)=20033​.

(iii) For x−3x^{-3}x−3:

33−5r=−3  ⟹  5r=3633-5r=-3 \implies 5r=3633−5r=−3⟹5r=36 Not an integer, so no such term.

  1. Therefore, the only contribution to the constant term is 33200.\boxed{\frac{33}{200}}.20033​​.

  2. Comparing with the stored correct answer: Stored answer is B, i.e. 33200\frac{33}{200}20033​, which matches our result.

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