A semi-circular gasket has an inner diameter of 4 cm and an outer radius of 4 cm. What is the area of the gasket in cm**2.
一个半圆环形垫片，它的内圆直径为4厘米，外圆半径为4厘米，这个垫片的面积是多少厘米**2．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">2 ** 2</gadget>
<output>4</output>

<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<gadget id="calculator">3.14 * 12</gadget>
<output>37.68</output>

<result>37.68</result>