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Binomial Theorem question

2022 · 29 Jun · Shift 1 · Q32
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  5. /2022 · 29 Jun · Shift 1 · Q32

Binomial Theorem question

2022 · 29 Jun · Shift 1 · Q32

JEE MainMathematicsBinomial TheoremMCQ+4 / −1
If the constant term in the expansion of (3x3−2x2+5x5)10{\left( {3{x^3} - 2{x^2} + {5 \over {{x^5}}}} \right)^{10}}(3x3−2x2+x55​)10 is 2k.l, where l is an odd integer, then the value of k is equal to:
  1. A
    6
  2. B
    7
  3. C
    8
  4. D
    9
View written solutionFree

Correct answer: D

  1. We need the constant term in (3x3−2x2+5x5)10.\left(3x^3-2x^2+\frac{5}{x^5}\right)^{10}.(3x3−2x2+x55​)10.

  2. In the general term of the multinomial expansion, if we choose:

  • aaa times the term 3x33x^33x3,
  • bbb times the term −2x2-2x^2−2x2,
  • ccc times the term 5x−55x^{-5}5x−5,

then a+b+c=10a+b+c=10a+b+c=10 and the corresponding term is 10!a!b!c!(3x3)a(−2x2)b(5x−5)c.\frac{10!}{a!b!c!}(3x^3)^a(-2x^2)^b(5x^{-5})^c.a!b!c!10!​(3x3)a(−2x2)b(5x−5)c.

  1. For this term to be constant, the power of xxx must be zero: 3a+2b−5c=0.3a+2b-5c=0.3a+2b−5c=0.

So we solve a+b+c=10a+b+c=10a+b+c=10 3a+2b−5c=0.3a+2b-5c=0.3a+2b−5c=0.

  1. Substitute c=10−a−bc=10-a-bc=10−a−b into the second equation: 3a+2b−5(10−a−b)=03a+2b-5(10-a-b)=03a+2b−5(10−a−b)=0 3a+2b−50+5a+5b=03a+2b-50+5a+5b=03a+2b−50+5a+5b=0 8a+7b=50.8a+7b=50.8a+7b=50.

Now find non-negative integer solutions.

Testing values of aaa:

  • If a=1a=1a=1, then 8+7b=50⇒7b=42⇒b=68+7b=50\Rightarrow 7b=42\Rightarrow b=68+7b=50⇒7b=42⇒b=6. Then c=10−1−6=3.c=10-1-6=3.c=10−1−6=3.

This gives the valid solution (a,b,c)=(1,6,3).(a,b,c)=(1,6,3).(a,b,c)=(1,6,3).

No other non-negative integer solution works.

  1. Therefore the constant term is 10!1!6!3!(3)1(−2)6(5)3.\frac{10!}{1!6!3!}(3)^1(-2)^6(5)^3.1!6!3!10!​(3)1(−2)6(5)3.

Now compute: 10!1!6!3!=10⋅9⋅8⋅73⋅2⋅1=840,\frac{10!}{1!6!3!}=\frac{10\cdot 9\cdot 8\cdot 7}{3\cdot 2\cdot 1}=840,1!6!3!10!​=3⋅2⋅110⋅9⋅8⋅7​=840, (−2)6=64.(-2)^6=64.(−2)6=64.

Hence constant term =840⋅3⋅64⋅125.=840\cdot 3\cdot 64\cdot 125.=840⋅3⋅64⋅125.

First, 64⋅125=8000.64\cdot 125=8000.64⋅125=8000. So 840⋅3⋅8000=2520⋅8000=20,160,000.840\cdot 3\cdot 8000=2520\cdot 8000=20,160,000.840⋅3⋅8000=2520⋅8000=20,160,000.

  1. We are told this constant term is of the form 2k⋅l2^k\cdot l2k⋅l, where lll is odd.

Now factorize: 20,160,000=2520⋅8000.20,160,000=2520\cdot 8000.20,160,000=2520⋅8000. Also, 2520=23⋅32⋅5⋅7,2520=2^3\cdot 3^2\cdot 5\cdot 7,2520=23⋅32⋅5⋅7, 8000=26⋅53.8000=2^6\cdot 5^3.8000=26⋅53. So, 20,160,000=23+6⋅(32⋅54⋅7)=29⋅l,20,160,000=2^{3+6}\cdot (3^2\cdot 5^4\cdot 7)=2^9\cdot l,20,160,000=23+6⋅(32⋅54⋅7)=29⋅l, where lll is odd.

Thus, k=9.k=9.k=9.

  1. Therefore the correct option is 9.\boxed{9}.9​.
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