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Devi's square — eight triangles
Devi has a square of paper of side 23 cm. She cut along the dotted lines to get one small square of area 49 cm² and 8 identical right-angled triangles. Triangle PQR is one such triangle. Find the length of PQ.
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About this method
Work out where the pieces land after a fold, cut or stack.
In this one. The two legs of one triangle lie end to end along a side of the big square, so they add to 23, while the small square's side is their difference, 7 — sum and difference give both legs at once.
How to spot it
Words like 'folded', 'cut along the dotted line' or 'stacked' appear beside a shape, and the question asks for a length or angle that only makes sense once the pieces are in their new positions.
Common mistake
Children try to calculate the answer before working out where the pieces actually end up, skipping the visualisation step that the whole question depends on.
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