positive integral

例句：

1. 1.The sum of all positive integers is infinite.
   所有正整数的和是无限的。
2. 2. The area under the curve of a function is often calculated using the integral.
   函数曲线下的面积通常通过积分来计算。
3. 3.The integral of a function over a closed interval is equal to the area under the curve.
   一个函数在一个闭区间上的积分等于曲线下面积。