Line AC is perpendicular to the circle centered at O with radius 1 unit. The length of AC is 1.5 units.
Using the pythogoream thereom, OA is the square root of (1.5)^2 + 1^2 = 3.25.
The exact answer is sqrt(3.25) - r = sqrt(3.25) - 1, and it's about 0.80
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