यदि $$y=\left[x+\sqrt{x^{2}-1}\right]^{15}+\left[x-\sqrt{x^{2}-1}\right]^{15}$$ है, तो $$\left(x^{2}-1\right) \frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}+x \frac{\mathrm{d} y}{\mathrm{~d} x}$$ बराबर है :