第2个回答 2013-01-24
设n=x,n(n+1)(2n+1)=x(x+1)(2x+1)
(1)当x=3t,t∈Z,
x(x+1)(2x+1)=3t(3t+1)(6t+1)/3=t(3t+1)(6t+1)
(2)当x=3t+1,t∈Z,
x(x+1)(2x+1)=(3t+1)(3t+2)(6t+3)/3=3(3t+1)(3t+2)(2t+1)/3=(3t+1)(3t+2)(2t+1)
(3)当x=3t+2,t∈Z,
x(x+1)(2x+1)=(3t+2)(3t+3)(6t+5)/3=3(3t+2)(t+1)(6t+5)/3=(3t+2)(t+1)(6t+5)
得证。