There are ten balls numbered 1 to 10 in the box, and Xiao Ming takes out a total of nine balls from the box three times. If starting from the second time, the sum of the numbers of the balls taken out each time is twice the number of the previous one, what is the number of the balls that have not been taken out?
盒子里放有编号为1至10的十个球，小明先后三次从盒中共取出九个球．如果从第二次开始，每次取出的球的编号之和都是前一次的2倍，那么未取出的球的编号是多少．
<gadget id="calculator">1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10</gadget>
<output>55</output>

<gadget id="calculator">7 * 7</gadget>
<output>49</output>

<gadget id="calculator">55 - 49</gadget>
<output>6</output>

<result>6</result>