Build a cylindrical cistern with a depth of 3.5 meters and a bottom diameter of 4 meters. What is the area of the cistern in square meters?
修一个深3.5米，底面直径是4米的圆柱形蓄水池，这个蓄水池的占地面积为多少平方米．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">2 ** 2</gadget>
<output>4</output>

<gadget id="calculator">3.14 * 4</gadget>
<output>12.56</output>

<result>12.56</result>