笔者在2014年九年级数学一模考试中命制了这样一道试题:如图a,已知直线l1∥l2∥l3,且l1,l2之间的距离为1,l2,l3之间的距离为2,点A、C分别在直线l2,l1上,(1)利用尺规作出以AC为底的等腰△ABC,使得点B落在直线l3上(保留作图痕迹,不写作法);(2)若(1)中得到的△ABC为等腰直角三角形,求AC的长; The author in 2014 ninth grade mathematics a model test in the life of such a test questions: Figure a, known lines l1 // l2 // l3, and l1, l2 distance between 1, l2, l3 distance between 2, point A, C, respectively, on the straight line l2, l1, (1) the use of ruler to make the bottom AC-based isosceles △ ABC, so that the point B fell on a straight line l3 (keep traces of the plot, not written); (2) If Δ ABC obtained in (1) is an isosceles right-angled triangle, find the length of AC;