How did we conclude that the base of the triangle is also r ?
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AC is the radius of the circle and Y-axis is tangent to the circle.Which implies that AC is perpendicular to the Y-axis.
Similarly BC is perpendicular to the X-axis.
Angle AOB is right angle. Which implies angle ACB is also right angled.
Now the quadrilateral OACB must be either a rectangle or a Square.
In either case OB must be equal to AC(as both in a rectangle and in a square opposite sides are equal) . So OB = R
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