The cross-section of the cement pipe laying the sewer is a circular ring with an outer diameter of 80 cm and an inner diameter of 72 cm. Find the area of the cross section of the cement pipe.
铺设下水道的水泥管的横截面是个圆环形，外直径是80厘米，内直径是72厘米．求水泥管横截面的面积．
<gadget id="calculator">80 / 2</gadget>
<output>40</output>

<gadget id="calculator">40 ** 2</gadget>
<output>1_600</output>

<gadget id="calculator">72 / 2</gadget>
<output>36</output>

<gadget id="calculator">36 ** 2</gadget>
<output>1_296</output>

<gadget id="calculator">1_600 - 1_296</gadget>
<output>304</output>

<gadget id="calculator">3.14 * 304</gadget>
<output>954.56</output>

<result>954.56</result>