y=mx+c c is a normal to the ellipsex2/a2+y2/b2=1 if  c2  is equal to

Equation of a normal to the ellipse x2/a2+y2/b2=1 is axcos⁡θ−bysin⁡θ=a2−b2  , if it represents mx−y=−c then amcos⁡θ=bsin⁡θ=−a2−b2c⇒ cos⁡θ=acma2−b2,sin⁡θ=−bca2−b2⇒ c2a2−b22a2m2+b2=1⇒ a2−b22m2a2+b2m2