(1)当n=1时,左=1,右=1,左=右成立;
假设当n=k时,1+3+6+.k(k+1)/2=1/6 k(k+1)(k+2)成立;
当n=k+1时,1+3+6+.k(k+1)/2+(k+1)(k+2)/2=1/6k(k+1)(k+2)+(k+1)(k+2)/2
=1/6(k+1)(k^2+2k+3k+6)
=1/6(k+1)(k^2+5k+6)
=1/6(k+1)(k+2)(k+3)
=右;
其余两个有问题吧!
(1)当n=1时,左=1,右=1,左=右成立;
假设当n=k时,1+3+6+.k(k+1)/2=1/6 k(k+1)(k+2)成立;
当n=k+1时,1+3+6+.k(k+1)/2+(k+1)(k+2)/2=1/6k(k+1)(k+2)+(k+1)(k+2)/2
=1/6(k+1)(k^2+2k+3k+6)
=1/6(k+1)(k^2+5k+6)
=1/6(k+1)(k+2)(k+3)
=右;
其余两个有问题吧!