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Irrational Arithmagons

Age 16 to 18
Challenge Level Yellow starYellow star
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In a multiplication arithmagon, the number on each edge of the arithmagon is the product of the numbers at the adjacent vertices.

You may wish to look at Multiplication Arithmagons and develop a general strategy for working out the numbers at the vertices, given the edge numbers, before tackling the question below.

 
Can you work out the numbers that belong in the circles to make this multiplication arithmagon correct?
arithmagon with -6 + 2 root 2, 7 + 7 root 2, 2 - 4 root 2

 
 If you're not sure where to start, click for a series of hints:

Each circle will be of the form $a+b\sqrt2$ for some values of $a$ and $b$. 
 
In general, a multiplication arithmagon can be solved by multiplying, dividing, and square-rooting.
 
Dividing an expression by $a+b\sqrt2$ is the same as saying, "What must I multiply $a+b\sqrt2$ by to get that expression?"
 
To find the square root of an expression such as $12-8\sqrt2$, consider the equation $(x+y\sqrt2)^2=12-8\sqrt2$, expand the brackets, and deduce $x$ and $y$.
 
 


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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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