Problem solved. I bought 2 new adapters, Dell branded and unbranded, and they both work perfectly. Even an adapter is connected to the vga monitor with a 10 meter extension have a nice day
Hello, sorry for my english. I have a Optiplex micro 5070 with 2 displayport.  i just bought a displayport to vga adapter to connect the monitor, but it doesn't work The adapter is working with other PC. So i think, it's not compatible. What can i do? I couldn't find the original Dell adapter. what kind of adapter should i buy? Thanks  
optiplex 5070 displayport to VGA adapter