一定的英文是fixed。
例句：
1、他们发行一定数量的可公开交易的股票。
They issue a fixed number of shares that trade publicly.
2、学习、工作都有一定的时间。
There are fixed times for study and for work.
3、勘探队员成天在野外作业，没有一定的住处。
The prospectors are always out in the field and never settle down in a fixed place.
4、当透镜的球差一定时，N值的改变将影响轴上最佳聚焦点的位置。
For the lens with a fixed SA, any change of N will also affect the best focal position.
5、并在此基础上对距黑体一定距离处探测器表面的辐照度值E进行了研究。
Based on this, irradiance about the surface of detector which has a fixed distance to the blackbody is researched.
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