主题：【求解】儿子的数学题，10年级，我不会。 -- 李侠东

脸红，来此请教，多谢。

A semi-circle of radius 8 cm, rocks back and forth along a line. The distance between the line on which the semi-circle sits and the line above is 12 cm, As it rocks without slipping, the semi-circle touches the line above at two points. ( When the semi-circle hits the line above it immediately rocks back in the other directions). The distance between these two points, in millimetres, is closest to (A) 55 (B) 53 (C)51 (D) 49 (E) 47