In a circle, the length of the arc whose central angle is 90° is the length of the circumference (fill in a fraction).
在一圆中，圆心角是90°的圆弧长是圆周长的多少(填几分之几)．
<gadget id="calculator">90 / 360</gadget>
<output>1/4 = around 0.25</output>

<result>1/4 = around 0.25</result>