What is the maximum number of points of intersection of 8 straight lines on the plane?
平面上8条直线最多有多少个交点．
<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<result>28</result>