A circular piece of iron has an outer radius of 7 cm and an inner radius of 0.5 decimeters. What is the area of the ring in square decimeters?
一个环形的铁片，外圆半径是7厘米，内圆半径是0.5分米，这个环形的面积是多少平方分米？
<gadget id="calculator">7 / 10</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">(7/10) ** 2</gadget>
<output>49/100 = around 0.49</output>

<gadget id="calculator">0.5 ** 2</gadget>
<output>0.25</output>

<gadget id="calculator">(49/100) - 0.25</gadget>
<output>0.24</output>

<gadget id="calculator">3.14 * 0.24</gadget>
<output>0.7536</output>

<result>0.7536</result>