在已有的研究和实践中，其成本有效性问题往往被忽视。
However, the cost effectiveness problem was often ignored in previous researches and practice.
在综述中，对已有的文献进行了简单评述，并指出了本文的研究方向。
In the summary, we make some simply review, and point out the study direction.
已有的对网球教学的研究大多是针对教学系统中的某些因素进行的，很少有人把网球教学作为一个整体来考虑。
Most research on tennis teaching is in progress for some factor. Few people consider the tennis teaching as a whole.
已有的对网球教学的研究大多是针对教学系统中的某些因素进行的，很少有人把网球教学作为一个整体来考虑。
Most research on tennis teaching is in progress for some factor. Few people consider the tennis teaching as a whole.
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