The cross section of a steel pipe is circular. The inner circle has a radius of 4 cm and the outer circle has a diameter of 10 cm. What is the cross-sectional area of the tube in square centimeters?
一根钢管的横截面是环形．内圆半径4厘米，外圆直径10厘米．钢管的横截面积多少平方厘米？
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">3.14 * 16</gadget>
<output>50.24</output>

<gadget id="calculator">78.5 - 50.24</gadget>
<output>28.26</output>

<result>28.26</result>