multiplication by variables

例句：

1. 1. The formula for the area of a circle is πr², where r is the radius and π is a constant.
   圆的面积公式是πr²，其中r是半径，π是一个常数。
2. 2. In economics, the concept of marginal utility refers to the additional satisfaction gained from consuming an additionalunit of a good or service.
   在经济学中，边际效用的概念指的是消费额外一单位商品或服务时获得的额外满足感。
3. 3. The multiplication by variables can be used to solve problems involving multiple variables and their interactions.
   乘法变量可以用来解决涉及多个变量及其相互作用的问题。