X Error of failed request: BadMatch (invalid parameter attributes)
Major opcode of failed request: 140 (RANDR)
Minor opcode of failed request: 18 (RRAddOutputMode)
Serial number of failed request: 33
Current serial number in output stream: 34
(values after xrandr --newmode to be pasted from second line of generated values with cvt command from your own computer, don't copy from here. In other words, after writing xrandr --newmode in terminal, copy the output of cvt command from its second line excluding word Modeline upto its end and paste it after xrandr --addmode. )
xrandr --addmode VGA1 "1600x900_60.00"
After these steps, resolution of 1600x900 will appear under the list of various resolutions available in display settings. This will remain listed for one session only. When computer is restarted, this will disappear. To make it appear permanently, we need to configure xorg.conf file and make this entry manually