题目

Not until the early years of the 19th century          what heat is. A. man did know                    B. man know  C. didn’t man know                D did man know   答案：D1．如图，△ABC内接于⊙O，（1）作弦BC的弦心距OD（要求：用尺规作图，保留作图痕迹）；（2）若∠A=60°，AB=AC=2，求OD的长．