>In my laptop, I can use Function F4 to switch modes to display on a projector. So, the first mode is the normal mode. The second mode sends the image to the projector. The third mode sends the image to the laptop and the projector. This however adjusts the laptop resolution to fit the projector resolution. All is ok so far.
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>My friend has an Acer. With that laptop, we cannot find a way to display on the projector. We tried with my laptop. All is ok. When we try with his laptop, using Function F5, which I think is the key to switch display mode, nothings happen. I believe there is a setting that locks it to the laptop only. Anyone would know what we would have to look for to resolve that issue?

I agree with Bill - I once troubleshot an Acer laptop with the same issue. Turned out it was a setting deep in Display settings, IIRC an ATI-specific setting on its own tab. I don't recall exactly what I had to reconfigure but I know the interface was non-standard, you had to click on the picture of the configuration option you wanted in order for it to be selected.

Sorry I can't be more specific but if you check out all of the various display properties, you should be able to find it with a little experimentation.