a) \({\cot ^2}x - {{2x\cos x} \over {{{\sin }^3}x}}\) ;
b) \({{\cos \sqrt x \cos 3x + 6\sqrt x \sin \sqrt x \sin 3x} \over {2\sqrt x {{\cos }^2}3x}}\) ;
c) \(6\cos 2x{\left( {\sin 2x + 8} \right)^2}\) ;
d) \(6{x^2}\tan x + {{2{x^3} - 5} \over {{{\cos }^2}x}}.\)