By definition, an algebraic group $G$ is reductive if its unipotent radical is $\{e\}$. The radical of an algebraic group is the identity component of its maximal normal solvable subgroup. The unipotent radical of $G$ is the set of all unipotent elements in the radical of $G$. Is there a general method to compute radical and unipotent radical of $G$?
We know that $GL_n$ is reductive so the unipotent radical of $GL_n$ is $\{e\}$. In particular, how to compute the radical and unipotent radical of $GL_n$? Thank you very much.