1. Subtract the radius of the small circle from the given arc radius (a-b).

2. Subtract the radius of the large are from a-b. 

3. From the centre of the small circle lightly swing (draw) an arc with radius s-b. 

4. From the centre of the large circle swing (draw) a large arc with radius L-B. 

5. From the intersection o lightly draw lines through the centres of each circle extended to the circumferences of each circle. This will establish the points of tangency of the required arc. 

6. Using o as a centre and o-t as a radius, lightly draw an arc  t-t and extend it around to the left and right about the centre o. 

7. Darken the required arc. 

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