步骤有两个：
1. 配置/etc/X11/xorg.conf（如果没有此文件可以创建）
Section "Screen"
Identifier "Default Screen"
DefaultDepth 24
SubSection "Display"
Virtual 3040 2140
EndSubSection
EndSection
2. 调用xrandr命令：
Screen 0: minimum 320 x 200, current 3040 x 1050, maximum 3040 x 2140
LVDS connected 1360x768+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
1366x768 60.0 +
1360x768 60.0*
1280x768 60.0
1280x720 60.0
1024x768 60.0
1024x600 60.0
800x600 60.0
800x480 60.0
640x480 60.0
DFP1 disconnected (normal left inverted right x axis y axis)
DFP2 disconnected (normal left inverted right x axis y axis)
DFP3 disconnected (normal left inverted right x axis y axis)
DFP4 disconnected (normal left inverted right x axis y axis)
DFP5 disconnected (normal left inverted right x axis y axis)
DFP6 disconnected (normal left inverted right x axis y axis)
DFP7 disconnected (normal left inverted right x axis y axis)
DFP8 disconnected (normal left inverted right x axis y axis)
DFP9 disconnected (normal left inverted right x axis y axis)
DFP10 disconnected (normal left inverted right x axis y axis)
DFP11 disconnected (normal left inverted right x axis y axis)
DFP12 disconnected (normal left inverted right x axis y axis)
CRT1 connected 1680x1050+1360+0 (normal left inverted right x axis y axis) 509mm x 286mm
1920x1080 60.0 +
1680x1050 60.0*
1280x1024 75.0 60.0
1440x900 75.0 59.9
1280x720 60.0
1024x768 75.0 60.0
800x600 75.0 60.3
640x480 75.0 72.8 67.0 59.9
在命令行里执行：
xrandr --output CRT1 --right-of LVDS --mode 1680x1050
其中，CRT1代表的是扩展屏幕，LVDS代表的是laptap的屏幕。
如果还需要其他命令，可以man xrandr
1、xrandr --output CRT1 --auto 开启外接显示器。（如果不成功，就reboot下）
2、xrandr --output CRT1 --auto --right-of LVDS --mode 1024x768 把外接显示器扩展到本本显示器的右侧