Wind the copper wire on the coil. If the diameter of the coil is 3 cm, how long is the minimum length of copper wire required to wind 200 turns?
将铜丝绕在线圈上，如果线圈的直径是3厘米，绕200圈至少需要多少长的铜丝？
<gadget id="calculator">200 / 100</gadget>
<output>2</output>

<gadget id="calculator">3.14 * 3 * 2</gadget>
<output>18.84</output>

<result>18.84</result>