Doubling and dislocation subtraction of geometric sequence: clever calculation 1-(4/5)-(4/25)-(4/125)-...-(4/(5**0))= how many.
等比数列扩倍与错位相减：巧算1-(4/5)-(4/25)-(4/125)-…-(4/(5**0))=多少．
<gadget id="calculator">5 - 4 - 1</gadget>
<output>0</output>

<gadget id="calculator">5 ** 0</gadget>
<output>1</output>

<gadget id="calculator">4 / 1</gadget>
<output>4</output>

<gadget id="calculator">0 + 4</gadget>
<output>4</output>

<gadget id="calculator">4 / 4</gadget>
<output>1</output>

<result>1</result>