Tr=3 Tr−1+6r,r=2,3,4,…nT2=3.T1+62 T2=3.6+62...(1) T3=3 T2+63 T3=3 T2+63 T3=3(3.6+62)+63 T3=32.6+3.62+63...(2) Tr=3r−1⋅6[1+36+(36)2+…+(36)r−1]Tr=3r−1⋅6(1+2+22+…+2r−1)Tr=6⋅3r−11⋅(−1)(1−2r)Tr=6⋅3r−1⋅(2r−1)Tr=36⋅3r⋅(2r−1) Tr=2⋅(6r−3r)Sn=2Σ(6r−3r)Sn=2⋅[56.(6n−1)−23.(3n−1)]Sn=2[1012(6n−1)−15(3n−1)]Sn=53[4⋅64−5⋅3n+1]∴n2−12n+39=3n2−12n+36=0n=6