f(x)=x^2-2x+3=（x-1)^2+2
函数在1处最小,左侧减少,右侧增加.
当1 在[t,t+1]内时,即0<=t<=1时,g(t）=2
当t>1,f(x)在[t,t+1]上增加,g(t)=t^2-2^t+3
当t<0,f(x)在[t,t+1]上减少,g(t)=（t+1）^2-2^（t-1）+3=t^2+2