1. It is still a unsolved problem that semi - rigid base appears the shrinking crack at early stage. 半刚性基层早期出现收缩裂缝，是目前尚未很好解决的施工技术问题。
2. The vastness of the oceans and the complexity of their problems still leave many puzzles unsolved. 浩瀚的海洋及其问题的复杂性仍然留下许多未解决的难题。
3. When students are forced to develop their own approaches to unsolved problems, they benefit much more than from exposure to the mathematics alone. 当一个学生被迫大展身手去对付未解决的问题时，比仅仅学习数学知识，其受益要多得多。——斯潘尼尔。
