There is a semicircle, its arc AB is known to be 12.56 cm long, then how long is its diameter AB in cm.
有一个半圆形，已知它的弧AB长12.56厘米，那么它的直径AB长多少厘米．
<gadget id="calculator">2 / 3.14</gadget>
<output>0.636943</output>

<gadget id="calculator">12.56 * 0.636943</gadget>
<output>8.000004</output>

<result>8.000004</result>