计算1-1/3+1/5-1/7+1/9-1/11.......+1/21的值

方法一：
package com.Summer_0424.cn;

/**
 * @author Summer
 * 计算1-1/3+1/5-1/7+1/9-1/11.......+1/21
 */
public class Test09 {

    public static void main(String[] args) {
        double sum = 0.0;
        int op = 1;//作为符号变化的参数，每循环一次，符号变化一次
        for (int i = 1; i <= 21; i+=2) {
            sum = sum + (1.0/i*op);//因为i为整数，计算结果为整数，所以用1.0
            op = -op; 
        }
        System.out.println(sum);
    }
    
}

方法二：

package com.Summer_0424.cn;

public class Test05 {

    public static void main(String[] args) {
        double sum1 = 0.0;
        double sum2 = 0.0;
    
        for (int i = 1; i <=21; i=i+4) {
            sum1 = sum1+1.0/i;
        }
        for (int j = 3; j <=19; j=j+4) {
            sum2=sum2+1.0/j;
        }
        sum2 = -sum2;
        double sum = sum1+sum2;
        System.out.println(sum);

    }

}