f(x)=-x^2+2ax
0<=x<=1
当0<=x<=1<=a,
f（x）min=0,
f(x)max=2a-1
当a<=0<=x<=1
f（x）min=2a-1,
f(x)max=0
当0<=a<=1/2
f（x）min=2a-1,
f(x)max=-a^2+2a^2=a^2
当1/2<=a<=1
f（x）min=0,
f(x)max=-a^2+2a^2=a^2