Cut a circular piece of paper and assemble it into an approximate rectangle whose width is equal to the radius and whose area is constant. The radius of this circle is 5 cm. What is the area of the approximate rectangle?
把一个圆形纸片剪开，拼成一个宽等于半径，面积不变的近似长方形，这个圆的半径是5厘米，近似长方形的面积是多少．
<gadget id="calculator">3.14 * 5</gadget>
<output>15.7</output>

<gadget id="calculator">15.7 * 5</gadget>
<output>78.5</output>

<result>78.5</result>