Topics that all come together:
- Parallel lines (in 3D)
- Alternate/Complementary/Supplementary etc angles
- Polygon properties
- Trigonometric ratios
- Pythagoras' theorem
- Sine Rule
- Cosine Rule
- Oblique and isometric drawing
- 3d visualisation
- Volume, Surface area
- Algebra (in all its guises)
My students have seen some success in this topic through the use of "wireframes" (made of skewers and connected with a hot glue gun) and templates placed in the wireframes to indicate various right angled and non right angled triangles. It also helps them see parallel lines and equivalent lengths/angles. This seems to be the concrete element they need to put all the other stuff in place.
I also try to introduce the topic multiple times throughout the year as new concepts are introduced (eg. parallel lines - > volume -> pythagoras -> trig ratios -> sine rule -> cosine rule)
Also useful is the discussion on solving triangles here.