n偑帺慠悢偺偲偒丄師偺摍幃傪悢妛揑婣擺朄偱徹柧偣傛丅
1񑓛+2񑧈+3񑺵+ゥ�+n(n+1)(n+2)=(1/4)n(n+1)(n+2)(n+3)

偍婅偄偟傑偡丅