Use a rectangle with a length of 10 cm and a width of 4 cm to cut out the largest semicircle. What is the area of the semicircle?
用一个长10厘米，宽4厘米的长方形，剪一个最大的半圆，这个半圆的面积是多少．
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<gadget id="calculator">3.14 * 8</gadget>
<output>25.12</output>

<result>25.12</result>