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UKMT Hamilton Olympiad, 2017

Geometry Difficulty 3.0 AMC 10/12 Prove it United Kingdom

The diagram shows squares $ABCD$ and $EFGD$ . The length of $BF$ is 10 cm. The area of trapezium $BCGF$ is 35 $\text{cm}^2$ . What is the length of $AB$ ?

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