Fill in the numbers according to the law: 1, (2/9), (3/25), (4/49), how much.
按规律填数：1，(2/9)，(3/25)，(4/49)，多少．
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">9 ** 2</gadget>
<output>81</output>

<gadget id="calculator">5 / 81</gadget>
<output>5/81 = around 0.061728</output>

<result>5/81 = around 0.061728</result>