It should be set to an arithmetic expression that has an integer as a result. 它设置了一个完整的数学表达应该能得到一个完整的结果。
For such a step to be firm, one would have to understand the basic concepts of mechanics and their mathematical expressions. 为使这一步坚实,必须理解力学的基本概念,以及它们的数学表达形式。