1. It is still a unsolved problem that semi - rigid base appears the shrinking crack at early stage. ����Ի������ڳ��������ѷ죬��Ŀǰ��δ�ܺý����ʩ���������⡣
2. The vastness of the oceans and the complexity of their problems still leave many puzzles unsolved. ��嫵ĺ���������ĸ�������Ȼ��������δ��������⡣
3. When students are forced to develop their own approaches to unsolved problems, they benefit much more than from exposure to the mathematics alone. ��һ��ѧ�����ȴ�չ����ȥ�Ը�δ���������ʱ���Ƚ���ѧϰ��ѧ֪ʶ��������Ҫ��öࡣ����˹�������