Hexy-Metry

A hexagon, with sides alternately a and b units in length, is inscribed in a circle. How big is the radius of the circle?
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Problem

Hexy-metry printable sheets: main problem , idea cards

 

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Hexy-Metry
A hexagon is inscribed inside a circle.
 
The sides of the hexagon are alternately $a$ and $b$ units in length.
 
What is the radius of the circle?