Dell Inspiron 3700 Reference Manual page 56

If the external monitor is turned off when you boot your computer, the computer still sends the video image to the external monitor. In this case, you
will not see an image on either the computer's display or the external monitor. To see an image, turn on the external monitor or switch the video
image to the computer's display by pressing <Fn><F8> on the keyboard or <Scroll Lock><F8> on an external keyboard if the External Hot Key
option is enabled in the System Setup program.
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