1、设A(x1,y1),B(x2,y2)
K=(y1-y2)/(x1-x2)=a-(x1^2+x2^2)(x1+x2)
又x∈[1/2,1],1/2≤k≤4
所以a-(1+1)(1+1)>=1/2且a-(1/4+1/4)(1/2+1/2)