X 2+Y 2-2x = 0, namely X 2+Y 2-2x+ 1 = 1, which is simplified as (X- 1) 2+Y 2 = 1. So the circle takes (1, 0) as the center and 1 as the radius. Because the straight line is tangent to the circle, the distance from the straight line to the center of the circle is radius 1. That is | (1+a)+1|/√ (1+a) 2+1) =1. So ((1+a)+1) 2 = (1+a) 2+1,2 (1+a) = 0. Therefore, A=- 1.