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You can delete apps that came from the Mac App Store. You can't delete ones that came from other sources. And keep in mind that if you delete an app from launchpad it will delete the actual app from your machine.
 
^^ What this guy said. This is the reason I'm not using Launchpad until Apple changes Launchpad for the better. The launchpad should be more like the dock, where it's only an alias that is placed. It would make LP a whole lot easier and better to use.
 
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