Equation of normal of the parabola y2=4a(x+c) isy=m(x+c)−2am−am3⇒y=mx+mc−2am−am3Similarly, equation of normal for the parabola y2=4bx is:y=mx−2bm−bm3As the normal is common, hencemc−2am−am3=−2bm−bm3(a−b)m3=m(c+2b−2a)⇒m2=a−bc+2b−2a=a−bc−2As m2>0, hence a−bc−2>0⇒c>2(a−b)