f(x)=1/[x^2-x+1]x^2-x+1=(x-1/2)^2+3/4最小值为3/4最大值为4/3
f(x)=1/[1-x(1-x)] =1/[x²-x+1]x²-x+1 有最小值 (4-1）/4=3/4所以 f(x) 有最大值 4/3
