For example, in physics, time is T, mass is M, and length is X, S, L, D, etc. The period is T, the current is I, the voltage is U, the capacitance is C, the resistance is R, the magnetic field is B, and the electric field is E ... In many cases, the initials of English words are used as unknown.

Mathematically, when an unknown is set, X is often used. If there are multiple unknowns, such as x, y, z, a, b, c, etc. Greek letters such as α, β and γ are also used to indicate angles.

In short, in institutions of higher learning, the unknown will have more meaning.