The radius of circle O is the same as half of the hypotenuse of the right triangle.
The find the hypotenuse, simply use the following,
![\sqrt{a { }^{2} + b ^{2} }](https://tex.z-dn.net/?f=%20%5Csqrt%7Ba%20%7B%20%7D%5E%7B2%7D%20%2B%20b%20%5E%7B2%7D%20%7D%20)
Or the square root of one leg squared us the other leg squared.
I already know this to be a 5 - 12 - 13 right triangle and you can get 13 with the equation.
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Half of 13 is 6.5 meaning 6.5 is our radius.
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I hope that helps you out!!
Any more questions, please feel free to ask me and I will gladly help you out!!
~Zoey