Index of (r+1)th Term from End When Counted from Beginning
If the coeffi...
Question
If the coefficient of 2nd, 3rd & 4th terms in the expansion (1+x)2n are in A.P., which of the following hold good
A
n2−9n+7=0
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B
2n2−9n−7=0
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C
2n2−9n+7=0
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D
9n2−7n+2=0
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Solution
The correct option is C2n2−9n+7=0 Hence 2nC1, 2nC2 and 2nC3 are in A.P. Therefore It must follow that (2n−2(2))2=(2n+2) (2n−4)2=2(n+1) 4(n−2)2=2(n+1) 2(n2−4n+8)=n+1 2n2−8n+8=n+1 2n2−9n+7=0