Q.
In the binomial expansion of (a-b)n,n≥5, the sum of 5th and 6th terms is zero, then a/b equals
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a
5n−4
b
6n−5
c
n-56
d
n-45
answer is D.
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Detailed Solution
Given T5+T6=0⇒C4 nan-4 b4-C5 nan-5 b5=0⇒C4 nan-4 b4=C5 nan-5 b5⇒an-4 b4an-5 b5=C5 nC4 n⇒ab=n-5+15⇒ab=n-45
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