Divide a circular piece of paper into a number of even-numbered parts and assemble it into an approximate rectangle. It is known that the length of the rectangle is 15.7 cm. What is the area of the circular piece of paper?
把一个圆形纸片平均分成若干偶数份，拼成一个近似的长方形，己知长方形的长是15.7cm，那这个圆形纸片的面积是多少．
<gadget id="calculator">15.7 / 3.14</gadget>
<output>5</output>

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

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

<result>78.5</result>