For cartesian plans, for functions, why are x and y the letters we use as variables?
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Descartes was the mathematician who proposed and formalized the coordinate system that was subsequently named after him (cartesian).
He used the first letters of the alphabet to signify known quantities and the last few letters to signify unknowns. The tradition started from there. (this might be apocryphal since x and y were probably already used before his time)
Very generally speaking, this developed into an informal ‘system’.
a,b,c are used for scalar (or real number) constants. (e isn’t typically used because it refers to euler’s constant. In physics, c, is usually reserved for the speed of light)
f,g,h are used for function names typically. In kinematics physics, g is usually reserved for the acceleration due to earth’s gravity
i,j (and sometimes k) is used as subscripts, indices or with a “hat” to denote unit vectors.
k,m,n, (mostly m and n) are used to denote integer constants or integer unknowns.
p,q,r are also commonly used to represent vector unknowns. (a,b,c are usually scalar constants or unknowns)
s,v and t are used in physics typically to denote displacement, velocity and time. (In kinematics physics, u is used for initial velocity)
u,v,w are also quite often used for vector variables.
x, y, z are used for variables. z is commonly used to represent complex number variables. (so if a problem states z^3 =1, you should assume that z refers to complex number solutions and responding with z = 1 is incomplete or incorrect.)
l and o are rarely used because it too easy to mistake it for “1” and “0”
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