There are 10 points on the paper, any three points are not on a line, how many straight lines can be drawn through two points.
纸上有10个点，任意三个点都不在一条线上，通过两点画直线可画多少条．
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">1 + 2 + 3</gadget>
<output>6</output>

<gadget id="calculator">1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9</gadget>
<output>45</output>

<result>45</result>