Draw 2 circles on a piece of paper with a maximum of 2 intersections; draw 3 circles with a maximum of 6 intersections; draw 4 circles of different sizes with a maximum of 12 intersections; ...; draw 7 circles of different sizes , how many intersection points there are at most.
在一张纸上画2个圆，最多有2个交点；画3个圆，最多有6个交点；画4个大小不同的圆，最多有12个交点；…；画7个大小不同的圆，最多有多少个交点．
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">7 * 6</gadget>
<output>42</output>

<result>42</result>