equations for lesson two are invisible due to githubs default dark theme

[beastr45]

black text on a black background. Just remove the alpha channel.

[DouYuexi]

我已经收到您的邮件了，我将会尽快处理，保持联系！

[digitsensitive]

You might think using this: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions