Which version of PowerPoint? For 2007 on home ribbon and drawing select chord and then grab one corner and move until you have the right angle (180 degrees) then shape outline and select dashes. I hope that is clear.
My advice will not work for PowerPoint 2003 which is why it is important to state which version. However it is not too different in that version How to draw a semi-circle. Having drawn it then you have to change the outline to dashed,
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