I need to build a triangular section on my benchwork fo 1 by 4s. It will have a right angle 90 degree triangle with the other angles of 20 degrees and 70 degrees. I have a good but basic miter saw but cannot for the life of me figure out how to cut a triangle and assemble from 1 by 4 . It would be a right triangle with sides of 48 inches, 17.5 inches and 52 inches with angles of 90 degrees, 20 degrees and 70 degrees. Any help appreciated.
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For what little overhang of the top will be the corners, there wouldnt be anything wrong with cutting 3 square ends on the boards and butting them up if you dont think you have the saw skills to get all the angles cut. Cutting a 70 is going to be a peach, as you'll have to put the board in the saw against a 45 degree stop and then adjust the saw another 25 degrees to get it, or cut both boards at a 37.5.
Just make 3 square boards add a brace from the hypotenuse to the opposite side and call it a day.
You can alternatively cut the top first, then trace the edges onto the 1x4s and adjust the saw to the trace lines, but you'll have to subtract 3/4" from your lines on one edge of each board for the overlap in each corner.
Try and lay it out in trace lines and see if makes more sense. If not, revert to the first paragraph.
I'd also probably consider making some small corner blocks of the right shape to give it strength in the corners.
Actually, what I do is ask @Tom Tee, he knows all the tricks. 
Here's one idea: take two 6" blocks of 2x4. Use the miter saw to cut one in half with a 70 degree angle, and the other at a 20 degree angle. Use each pair as inside blocks to screw the sides into, on at top and one at bottom of 1x4. Don't try to angle cut the edge of the 1x4; just butt the edges together.
Before you plan or cut anything, verify the accuracy/precision of your angle and length measurements using your high school trigonometry (sine is the ratio of opposite side to hypotenuse, for example) Verify you have a right triangle.
There might be a bigger issue there than you could induce hacking with a miter saw.
If you can't do the math, I'm sure someone will be able to help. You have more than enough information posted already.
As Boilermaker said, to get angles sharper than 45* use a 45 degree stop or scrap cut to 45 and secured to fence, then adust the saw the required amount.
When doing very sharp cuts you need to be carful that the work piece does not pull into the saw as there is sometimes a wide enough gap in the fence (depending on the saw) that there won't be anything to keep the piece in place. Adding a sacrificial wood fence is a good idea.
Do the more difficult cut first, if there is an error you can just recut.
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