计算序列的前n项和。。

    int ride = -1 ;
    double sum = 1.0;
    int count = 1;
    while(count < 3){
        int denominator = 1 + (3 *count);
        sum = sum+ (1.0/denominator )*ride;
        ride  = ride * (-1);
        count ++;
    }
    System.out.println(sum);