In a rectangle with a length of 7.8 cm and a width of 6.4 cm, draw the largest circle. What is the circumference of this circle?
在一个长7.8cm，宽6.4cm的长方形里，画一个最大的圆，这个圆的周长是多少．
<gadget id="calculator">3.14 * 6.4</gadget>
<output>20.096</output>

<result>20.096</result>