xn=1/2(x(n+1)+x(n-1))
(x(n+1)-xn)/2=(xn-x(n-1))/2
(xn-x(n-1))/2=(x(n-1)-x(n-2))/2
...
(x3-x2)/2=(x2-x1)/2
累加得 (x(n+1)-x2)/2=(xn-x1)/2
x(n+1)+3=xn-1
x(n+1)=xn-4
等差数列 d=-4 x1=1
通项xn=x1+(n-1)d=5-4n