Q.

If, in the binomial expansion of (a−b)n,n≥5 he sum of 5th and 6th term is zero, then value of ab is

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a

n−56

b

6n−4

c

n−45

d

5n−4

answer is C.

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Detailed Solution

nC4an−4(−b)4+nC5an−5(−b)5=0⇒nC4an−4b4=nC5an−5b5⇒an−4b4an−5b5= nC5 nC4⇒ab=n−45
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If, in the binomial expansion of (a−b)n,n≥5 he sum of 5th and 6th term is zero, then value of ab is