分享一道题:有数列：9，99，999，9999...，编程计算前10项的值。

public class Demo3{
public static void main(String[] args){
double sum = 0;
double sumnumber = 1;
for(int i = 1; i <= 10; i++){
sumnumber = sumnumber*10;
sum = sum + sumnumber -1;
}
System.out.println(sum);
}
}