A circular iron sheet has an inner radius of 5 cm and an outer radius of 7 cm. What is the area of the ring in square centimeters?
一个环形铁皮，内圆半径是5厘米，外圆半径是7厘米，环形铁皮的面积是多少平方厘米？
<gadget id="calculator">7 ** 2</gadget>
<output>49</output>

<gadget id="calculator">3.14 * 49</gadget>
<output>153.86</output>

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

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

<gadget id="calculator">153.86 - 78.5</gadget>
<output>75.36</output>

<result>75.36</result>