OK, my approach is to create a S90S triangle (https://www.talkgraphics.com/showthr...Triangles-S90S).
At this point I have the Shape tool open so I tab to the top node and change the -90 angle (A) to -38.


For S17S, change -90 to -163.
The trick is the entered value is -(180 - Angle), as identified by Marco.
With a changed Angle, a new triangle is available at any time, but necessarily requires a bit of arithmetic.