What is the area of a circle with a radius of 4 decimeters?
半径是4分米的圆，它的面积是多少平方分米．
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">3.14 * 16</gadget>
<output>50.24</output>

<result>50.24</result>