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Highly selective Backlog Qs for JEE MainMathematicsBinomial Theorem

If in the expansion of (a-2 b)^ n , the sum of the 5 th and 6 th term is zero, then the value of a b is

Options

  1. An -4 5
  2. B2(n-4) 5
  3. C5 n-4
  4. D5 2(n-4)

Correct answer

B. 2(n-4) 5

Step-by-step solution

Hints: gathered (a-2 b)^ n = _ r =0 ^ n ^ n C _ r (a)^ n - r (-2 ~b )^ r t ₅+ t ₆=0 gathered aligned & ^ n C ₄( a )^ n -4 (-2 ~b )^4+ ^ n C ₅( a )^ n -5 (-2 ~b )^5=0 n ! 4 !( n -4) ! a ^ n -4 (-2 ~b )^4=- n ! 5 !( n -5) ! ( a )^ n -5 (-2 ~b )^5 & 1 ( n -4) a = -1 5 (-2 ~b ) a b = 2( n -4) 5 aligned

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