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Display modes
Fn + This hot key cycles through the power-on display options.
The display modes are:
Built-in display panel only
Built-in display panel and external monitor
simultaneously
External monitor only
Built-in display panel and external video device
simultaneously
External video device only

Sample display options window

In order to use a simultaneous mode, you must set the
resolution of the internal display panel to match the
resolution of the external display device.