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'Of All the Areas' printed from http://nrich.maths.org/
You may find it useful to take a look at More Isometric Areas
These diagrams might help you to work out the areas of the tilted triangles.
The top diagrams show how drawing a non-tilted triangle around the tilted triangle could help you to find the area of the tilted triangle.
Area of tilted triangle = Area of blue triangle - Area of 3 green triangles
This diagram show some lines that can be drawn inside the tilted triangle:
Area of tilted triangle = Area of pink triangle + Area of 3 blue triangles