n为偶数时，tn=(1/2)t(n-1)=(5/4)(t(n-2)+1)(tn+5)=(5/4)(t(n-2)+5)=(5/4)²(t(n-4)+5)=…=(5/4)^((n-2)/2)(t2+5)t1000=(5/4)^499(t1/2+5)=(11/2)(5/4)^499
