OK, I really searched for an easy way to do this, but this technique seems to be the simplest. There are a few steps involved.
Choose your Rectangle Tool (Not the Rectangular Marquee Tool!)
On a new layer, create a rectangle by clicking once and holding on the upper center guidelines, start dragging, then add in and hold down the shift key and the Opt/Alt key at the same time. Drag until the rectangle is even with the lower horizontal guideline.
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On a new layer, you can repeat the same process on the left or you can duplicate the first rectangle using Cmd/Cntrl + J , and then use the Move Tool to position it to the left.
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Next, you need to select the Direct Selection Tool.
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Select the right side rectangle layer.
ON the canvas, click once on the right side rectangle. This will add a path with anchor points. Place the cursor on the lower right anchor point, right click and choose "Delete Anchor Point".
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Repeat for the left side rectangle.
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Almost there! LOL!
Turn off the two new triangle layers.
Select/Highlight the Gradient Arc layer.
Hold down the Cmd/Cntrl key, place your mouse cursor over the the right triangle layers thumbnail and click once.
This will make a selection of the right triangle........
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Now hold down the shift key and the Cmd/Cntrl key, place your mouse cursor over the the left triangle layers thumbnail and click once.
This will add the left side triangles selection to the rights.
Note: Only the new larger triangle is selected!
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Now hold down the shift key and the Cmd/Cntrl key, then hit I (Inverse)
Note: Now the selection is around the larger triangle and the outside of the document.....
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Now hit the delete key! Then Cmd/Cntrl + D to deselect. DONE!
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You can clear the guides by going to View > Clear Guides
