exercițiul 13 va rog mult

[tex]\it b=6c\ \ \ \ (1)\\ \\ a=2b\stackrel{(1)}{\Longrightarrow}\ a=12c \ \ \ \ (2)\\ \\ (1),\ (2) \Longrightarrow\ (a+b):c=(12c+6c):c=18c:c=18\\ \\ (1),\ (2) \Longrightarrow\ (a-b):c=(12c-6c):c=6c:c=6[/tex]