Unable to dual display Ubuntu using xrandr.

I use Ubuntu 14.04 by dual boot with Windows.I try to dual display on LVDS1 desktop and VGA1 external monitor, but it doesn't work.I usually use it on LVDS1, but when I connect it to an external display with a VGA connector, it doesn't show on LVDS1, and only VGA1 is shown.

If you disconnect the VGA connector and run xrandr, you will see the following results:

Screen 0: 8 minimum 8 , current 1280 x 1024, maximum 32767 x 32767
LVDS1 connected primary 1280x1024+0+0 (normal left inverted right x axial) 0mm x 0mm
1280x102459.7*+
1280x96060.0
1152x86460.0
1024x76860.0
800x600 60.356.2
640x48059.9 VGA1 disconnected (normal left inverted right x axial)
VIRTUAL1 disconnected (normal left inverted right x axial)

On the other hand, when you run xrandr on a VGA connector:

Screen 0: minimum 8 x 8, current 1440 x 900, maximum 32767 x 32767 LVDS1 disconnected (normal left inverted right x axial)
VGA1 connected primary 1440x900+0+0 (normal left inverted right x axial) 408mm x 255mm
1440x900 59.9+75.0*
1280x102475.060.0
1152x86475.0
1024x76875.170.160.0
832x62474.6
800x600 72.2 75.0 60.356.2
640x480 75.072.866.760.0
720x40070.1
VIRTUAL1 disconnected (normal left inverted right x axial)

I want to operate with the VGA connector connected and LVDS1 connected, but I don't know how to set it up.
I would appreciate it if someone could tell me.Thank you for your cooperation.