There are 10 students in the table tennis interest group of Yangcun Primary School competing with each other. How many times will there be a total of competitions? Use the method of drawing pictures or lists to find the rules, and then obtain the results.
阳村小学乒乓球兴趣小组有10名同学进行比赛，每两人比赛一次，一共要比赛多少次？用画图或列表的方法找规律，再求出结果．
<gadget id="calculator">4 + 3 + 2 + 1</gadget>
<output>10</output>

<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">10 * (9/2)</gadget>
<output>45</output>

<result>45</result>