If the radius of a circle is 4 cm, what is its area in square centimeters?
一个圆的半径是4厘米，它的面积是多少平方厘米．
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

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

<result>50.24</result>