1、n=4时，f(4)=2；2、n=5时，f(5)=5；3、假设，n=k时，有f(k)=0.5k(k-3)f（k+1）=f（k）+k-2=0.5k(k-3)+k+1-2=0.5(k^2-3k+2k-2)=0.5[(k^2-1)-(k+1)]=0.5(k+1)(k+1-3)
