3.6+6.9+9.12+....+3n(3n+1)n−timesNow,Tr=3r(3r+3)=9r2+9rSn=∑nr=1Tr=∑nr=1(9r2+9r)=9∑nr=1r2+9∑nr=1r=9n(n+1)(2n+1)6+9n(n+1)2=3n(n+1)(2n+1)2+9n(n+1)2=n(n+1)2[3(2n+1)+9]=n(n+1)2[6n+12]=(3n+6)n(n+1)=(3n)(n+1)(n+2)Hence,3.6+6.9+9.12+....+3n(3n+1)=(3n)(n+1)(n+2)proved.