As shown in the figure, it is known that △ ABC is inscribed in ⊙ o, ∠ C = 45 ° and ab = 4, then the radius of ⊙ o is___ ．

Connect OA, ob ∵ ∠ C = 45 ∵ AOB = 90 ° and ∵ OA = ob, ab = 4 ∵ OA = 22

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