In this post,we give a geometrical proof of the rearrangement inequality via dot product of two vectors. For vectors $\mathbf{OA}=(a_1,\cdots,a_n)$ and $\mathbf{OB}=(b_1,\cdots,b_n)$ in $\mathbf{R}^n$,the dot product of $\mathbf{OA}$ and $\mathbf{OB}$ is denoted by $\mathbf{OA}\cdot\mathbf{OB}$,w ...