If the length of the arc contained in a sector is (1/5) the length of the circumference of a circle with the same radius, then the area of the sector is equal to the area of the circle.
如果一个扇形所含圆弧的长是相同半径圆周长的(1/5)，那么这个扇形的面积是这个圆面积的多少．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<result>1/5 = around 0.2</result>