X and Y
These are two magical words we can find everywhere   These words are often used in science to represent any entities, especially in mathematical formulations.   I was wondering why differential equation often represent as dy/dx not da/db  ?  Why Male Chromosome is represented by xy  rather than ab ?  Why Female Chromosome is represented by xx  rather than aa  ?  Why random variables in probabilities and statistics theories generally represented by X , not R ?  Why function f( x ) is defined as the rule that map x  to y ?  Why don't we define function f( p ) as the rule that map p  to q  instead ?   Why two dimensional vector generally use x  and y  to represent the dimensions ?  Why don't we use a  and b  instead ?  Why the equation of linear regression is formulated as y =a x +b and not q =a p +b?   In daily life, x  is used to represent some mysterious or unknown entities  for example: Where the cashflow from Mr. X  goes to?  see?   Yet, it's used for some negative interp...