with the two diagonals
why must the orange (left) and yellow (right) triangles have the same area?
if the bottom (base) of the trapezium is double the length of the top,
and because the triangles are similar
what is the relationship between the areas of the top (gold) and bottom (light red) triangles?
[it is 4 times bigger]
so, what fraction of the whole trapezium area is the top (gold) little triangle?
what if the base is treble the top?
etc.
in general, if the bottom length is 'k' times longer than the top length,
show that the orange and yellow triangles are (each)
k times the area of the top (gold) triangle
[a bit tricky]
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