A question

Make use of chain rule to do it.

y = xn sinx + nxn-1 cosx

dy/dx = xn d(sinx)/dx + sinx d(xn)/dx + n[xn-1 d(cosx)/dx + cosx d(xn-1)/dx]

= xncosx + nxn-1sinx - nxn-1sinx + n(n - 1)xn-2cosx

= xncosx + n(n - 1)xn-2cosx

= xn-2(x2 + n(n - 1))cosx