当a>=1时 x∈[a,a+1]即x>=1
f(x)=(x-1)^2+1
此时当x=a时最小 g(a)=(a-1)^2+1
当a+1<=1时 x<=1
此时当x=a+1时最小 g(a)=a^2+1
0=
g(a)=1