《乱码篇》-傅里叶变换推导

参考：

https://zhuanlan.zhihu.com/p/41875010
https://zhuanlan.zhihu.com/p/108598607
https://www.leiphone.com/category/yanxishe/HJWOsm2lCtCIpVVK.html
https://www.geogebra.org/download