Let us first bring-in the { 3-4-5 Pythagorus-Triple Triangle } ,
- and identify the angle { Angle THETA } as being the { arctangent of [ 3-over-4 ] } ,
- as per the diagram on-the-left below .
Consequently , we can set up the new { Triangle A-B-C } in accordance to procedure as described in Part III of this paper ,
- but now using 2 { 3-4-5 Pythagorus-Triple Triangles } instead ;
- as per the diagram in-the-middle below .

But we take note at this point that :
- { Triangle B-O-C } is a { 15-20-25 Pythagorus-Triple Triangle } ,
- with [ Angle B-O-C ] being a right-angle of 90 degrees ;
- as per the diagram on-the-right above .