A ring has an inner radius of 5 cm and an outer radius of 7 cm, what is its area in square centimetres?
一个圆环，内圆半径是5厘米，外圆半径是7厘米，它的面积是多少平方厘米．
<gadget id="calculator">7 ** 2</gadget>
<output>49</output>

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

<gadget id="calculator">49 - 25</gadget>
<output>24</output>

<gadget id="calculator">3.14 * 24</gadget>
<output>75.36</output>

<result>75.36</result>