lean_workbook_plus_53656

Prove that |\begin{ matrix } -2a & \quad \quad \quad \quad a+b & \quad \quad \quad \quad c+a \ a+b & \quad \quad -2b & \quad \quad \quad \quad b+c \ c+a & \quad \quad \quad \quad c+b & \quad \quad \quad \quad -2c \end{ matrix }|=\quad \quad 4(a+b)(b+c)(c+a)