The central angle of the sector is doubled, and the radius is reduced to 1/2. At this time, the area of the sector is equal to the original area.
扇形的圆心角扩大到原来的2倍，半径缩小为原来的(1/2)，此时扇形的面积是原来面积的多少．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<result>1/2 = around 0.5</result>