A circle with a diameter of 20 cm increases its area by 10 cm when its diameter increases by 10 cm.
直径为20厘米的圆，直径增加10厘米后它的面积增加多少平方厘米．
<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">15 ** 2</gadget>
<output>225</output>

<gadget id="calculator">3.14 * 225</gadget>
<output>706.5</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

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

<gadget id="calculator">3.14 * 100</gadget>
<output>314</output>

<gadget id="calculator">706.5 - 314</gadget>
<output>392.5</output>

<result>392.5</result>