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Binomial Theorem question

2023 · 25 Jan · Shift 1 · Q39
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  5. /2023 · 25 Jan · Shift 1 · Q39

Binomial Theorem question

2023 · 25 Jan · Shift 1 · Q39

JEE MainMathematicsBinomial TheoremNumerical+4 / −1
The constant term in the expansion of (2x+1x7+3x2)5{\left( {2x + {1 \over {{x^7}}} + 3{x^2}} \right)^5}(2x+x71​+3x2)5 is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 1080

  1. We need the constant term in (2x+1x7+3x2)5.\left(2x+\frac{1}{x^7}+3x^2\right)^5.(2x+x71​+3x2)5.

  2. Let the terms selected from the 5 factors be:

  • aaa times 2x2x2x
  • bbb times x−7x^{-7}x−7
  • ccc times 3x23x^23x2

Then a+b+c=5.a+b+c=5.a+b+c=5.

  1. For the resulting term to be constant, the total power of xxx must be 000: a−7b+2c=0.a-7b+2c=0.a−7b+2c=0.

So we solve a+b+c=5,a−7b+2c=0a+b+c=5, \qquad a-7b+2c=0a+b+c=5,a−7b+2c=0 for non-negative integers a,b,ca,b,ca,b,c.

  1. From a=5−b−ca=5-b-ca=5−b−c, substitute into the second equation: 5−b−c−7b+2c=05-b-c-7b+2c=05−b−c−7b+2c=0 5−8b+c=05-8b+c=05−8b+c=0 c=8b−5.c=8b-5.c=8b−5.

Then a=5−b−(8b−5)=10−9b.a=5-b-(8b-5)=10-9b.a=5−b−(8b−5)=10−9b.

Since a,c≥0a,c\ge 0a,c≥0, check possible integer values of bbb:

  • If b=0b=0b=0, then c=−5c=-5c=−5 impossible.
  • If b=1b=1b=1, then c=3c=3c=3, a=1a=1a=1 valid.
  • If b≥2b\ge 2b≥2, then a=10−9b<0a=10-9b<0a=10−9b<0 impossible.

So the only valid choice is a=1,b=1,c=3.a=1,\quad b=1,\quad c=3.a=1,b=1,c=3.

  1. Hence the constant term comes from choosing:
  • one 2x2x2x
  • one 1x7\dfrac{1}{x^7}x71​
  • three 3x23x^23x2

Its coefficient is 5!1!1!3!(2)1(1)1(3)3.\frac{5!}{1!1!3!}(2)^1(1)^1(3)^3.1!1!3!5!​(2)1(1)1(3)3.

  1. Compute: 5!1!1!3!=1206=20,\frac{5!}{1!1!3!}=\frac{120}{6}=20,1!1!3!5!​=6120​=20, 2⋅33=2⋅27=54.2\cdot 3^3=2\cdot 27=54.2⋅33=2⋅27=54.

Therefore, constant term=20⋅54=1080.\text{constant term}=20\cdot 54=1080.constant term=20⋅54=1080.

  1. Comparison with stored answer:
  • Derived answer: 108010801080
  • Stored correct answer: 108010801080

They match.

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