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Question

In the binomial expansion of (ab)n,n5, the sum of the 5th and 6th term is zero. Then A/b equals

A
(n5)/6
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B
(n4)/5
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C
5/(n4)
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D
6/(n4)
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Solution

The correct option is B (n4)/5
(ab)nnC0an+...nCn(b)n
5thterm+6thtan=0
nC4an4(b)4+nC5an5(b)5=0
h!(n4)!4!an4b4n!(n5)!5!an5b5=0
n!(n4)(n5)!4!ana7.bn=n!(n1)!5×4!×ana5b5
1n4=15ba
ab=n45
Option B is correct

1142206_1143652_ans_cd0118ba0f114df1a2734d9c8ad8a5bf.jpg

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