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UKMT Senior Kangaroo, 2016

Number theory Difficulty 1.0 Junior Prove it United Kingdom

Using this picture we can observe that $1 + 3 + 5 + 7 = 4 \times 4$ . What is the value of $1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19 + 21$ ?

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