# [Jelly], 9 bytes

    ÷400⁵*÷S$

[Try it online!] or [View all test cases.]

The input is an array `[Ra, Rb]` and the output is an array `[Ea, Eb]`.

## Explanation

    ÷400⁵*÷S$  Input: array [Ra, Rb]
    ÷400       Divide each by 400, makes [Ra/400, Rb/400]
        ⁵*     Raise 10 to that power, makes [10^(Ra/400), 10^(Rb/400)]
            $  Monadic chain operating on previous result
          ÷      Divide each by
           S     The sum of the whole
               Makes [10^(Ra/400)/(10^(Ra/400) + 10^(Rb/400)),
                      10^(Rb/400)/(10^(Ra/400) + 10^(Rb/400))]
                   = [1/(1 + 10^((Rb-Ra)/400)), 1/(1 + 10^((Ra-Rb)/400))]

[Jelly]: https://github.com/DennisMitchell/jelly
[Try it online!]: https://tio.run/nexus/jelly#@394u4mBwaPGrVqHtwer/P//P9rQyMBAR8HI0MAgFgA "Jelly – TIO Nexus"
[View all test cases.]: https://tio.run/nexus/jelly#@394u4mBwaPGrVqHtwer/D/c/qhpjfv//9HRhkYGBjoKRoYGBrE6CtGGOgqGINoMJAYWsQQCHQUgYREbCwA