If the coefficients of rth and (r+1)th terms in the expansion of (3+7x)29are equal, then r equals

The general term Tr-1 in the expansion of (3+7x)29is Tr+1=29Cr329−r(7x)r= 29Cr×329−r×7rxr∴     ar = Coefficient of (r +1)th term =29Cr×329−r×7rNow,       ar=ar−1⇒ 29Cr×329−r×7r=29Cr−1×330−r×7r−1⇒  29Cr 29Cr−1=37⇒30−rr=37⇒r=21