y=x+t代入x^2/4+y^2=1
x^2+4(x+t)^2=4
5x^2+8tx+4t^2-4=0
|x1-x2|^2=(x1+x2)^2-4x1x2
=(8t/5)^2-4(4t^2-4)/5
=[64t^2-80t^2+80]/25
=[80-16t^2]/25
当t=0时
|x1-x2|^2达到最大为80/25
|AB|=√2|x1-x2|=4√10/5