Equation of tangent to parabola y2=8x at (2,−4) is −4y=4(x+2) ⇒x+y+2=0 ...(i) Center and radius of circle x2+y2=a is (0,0) and √a respectively. ∵ Eq. (i) is tangent to the circle. ⇒ Perpendicular distance of Eq. (i) from center (0,0)=√a ⇒|