The outer diameter of a ring is 7 cm and the inner diameter is 5 cm. What is the area of the ring in square centimetres?
一个圆环的外圆直径是7厘米，内圆直径是5厘米，这个圆环的面积是多少平方厘米．
<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">(7/2) ** 2</gadget>
<output>49/4 = around 12.25</output>

<gadget id="calculator">3.14 * (49/4)</gadget>
<output>38.465</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">(5/2) ** 2</gadget>
<output>25/4 = around 6.25</output>

<gadget id="calculator">3.14 * (25/4)</gadget>
<output>19.625</output>

<gadget id="calculator">38.465 - 19.625</gadget>
<output>18.84</output>

<result>18.84</result>