中学代数:增减辗转相除法 to find the remainder of f(x) when x= u
If f(u)=0, then x=u is a root of f(x) or (x-u) a factor of f(x).
In Abstract Algebra (Ring Theory since Polynomial has Ring structure behaves exactly like Integers), we note f(x)/(x-u) where (x-u) is the IDEAL of f(x).
Theory: f(x)=p(x).(x-u)+r(x) …(1) At x= u, (x-u)=0 f(u)= r(u) r(u) being the remainder.
If r(u)=0, from (1): f(u)=p(u).(x-u) then (x-u) is a factor (IDEAL) , or x=u is a root of f(x).
Algebraic Geometry is a study of all IDEALS of the polynomials f(x). Like study D24/猫山/黑刺 durians, just enough by analysing their kernel (核)。
Note: Idéal in “Ring” is similar to Kernel in “Group”. They are both the “essence” (aka “DNA”) of the structure Ring or Group, respectively.