How to extrude cut to make square cone - not equal sides

How to extrude cut to make square cone - not equal sides. I am trying to create a square cone that does not have equal sides, please see attached file. I started with a simple square and drew my square cone inside it, using 3-d sketch. Now, how do I extrude cut my coners off the square to make my cone?? Thanks, Gerry

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