If the side of a cone is unfolded into a fan shape, the central angle is 90°, and the radius is 20cm, what is the area of the bottom of the original cone?
若把一个圆锥的侧面展开后是一个扇形，且圆心角是90°，半径为20cm，则原来圆锥的底面积是多大？
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<result>78.5</result>