6th question
Q.6. If the coefficients of rth and (r + 1)th terms in the expansion of 3 + 7 x 29 are equal, then r equals
a. 15
b. 21
c. 14
d. none of these

Dear student

We know that the coefficient of rth in the expansion of 1+xn is nCr-1So, coefficients of rth and r+1th term in the expansion  of 3+7x29are 29Cr-1 and 29Cr+1-1i.e.  29Cr-1 and 29CrIt is given that these coefficients are equal29Cr-1 = 29Crr-1=r or r-1+r=29   as nCr=nCsr=s or r+s=n r-1=r  not possibleSo, r-1+r=292r=30r=15
Regards

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