There is a circle, r=10cm, R=15cm, what is its area in square decimeters?
有一个圆环，r=10厘米，R=15厘米，它的面积是多少平方分米．
<gadget id="calculator">15 ** 2</gadget>
<output>225</output>

<gadget id="calculator">10 ** 2</gadget>
<output>100</output>

<gadget id="calculator">225 - 100</gadget>
<output>125</output>

<gadget id="calculator">3.14 * 125</gadget>
<output>392.5</output>

<result>392.5</result>