Use a semicircular piece of paper with a diameter of 20 cm to make the side of a cone. What is the area of the base of the cone in square centimeters?
用直径20厘米的半圆形纸片，做一个圆锥的侧面，这个圆锥的底面积是多少平方厘米？
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">3.14 * 25</gadget>
<output>78.5</output>

<result>78.5</result>