Show that the equation of a straight line meeting the circle x?+y?=a? in two points at equal distance d from a point (x1,y1) on its circumference is xx1+yy1-a?+d?/2=0
Let S ≡ x2 + y2 − a2 = 0. See Fig. The required line is the common chord of the circle S = 0 and S' ≡ (x − x1)2 + (y − y1)2 + d2. 
