# If $\cos^{-1}\bigg(\large\frac{x^2-y^2}{x^2+y^2}\bigg)=k$ (a constant ), then $\large\frac{dy}{dx}=$

$(a)\;\frac{y}{x} \quad (b)\;\frac{x}{y} \quad (c)\;\frac{x^2}{y^2} \quad (d)\;\frac{y^2}{x^2}$

$(a)\;\frac{y}{x}$