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If in the expansion of the sum of the coefficients of then the coefficient of the third term is:
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a
455
b
105
c
605
d
120
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Correct option is D
Tr+1=nCrx3n−r−1x2r=nCr(−1)rx3n−5rFor coefficient of x5, set 3n−5r=5⇒r=(3n−10)/5=m(say)Note that l−m=1.We are given nCl(−1)l+nCm(−1)m=0⇒nCl=nCm⇒l+m=n.⇒3n−55+3n−105=n⇒6n−15=5n⇒n=15.coefficient of third term =15C2(−1)2=105.Similar Questions
If then equal to
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