When using a variable for "whatever coefficient you happen to have", since there is no standard, inserting "with coefficient 'a'" or something similar could be helpful for students who are lacking caffeine and/or sleep.
Phi rhymes with pie, and is (1+√5) / 2. Functions are to be designated by Latin Alphabetical Characters in a Script font, please. (Or should I discover some useful function, I will declare that function to be π(x)!)
Chi rhymes with tea. What statisticians say is spelled Kai or Khi. (A C followed by an H should never be a hard C) Its symbol is specious at best, as it can be mistaken for an 'X'. Since 'x' is quite often squared already, this really, really, really was a bad choice of characters to use.