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Term | 1 | 2 | 3 | 4 | 5 |
Area | 1 | 2 | 4 | 8 | 16 |
And they spotted that the $n^\text {th}$ term is equal to $2^{n-1}$.
The reason for this is because each trapezium is a similar shape to the one before. So, if we increase by scale factor $\sqrt2$ (due to the relative sizes of a right angled isosceles triangle), the area factor will increase by 2. Because $AF = SF^2$.