Let f (x) =

a

x4

−

b

x2

+ cos x
⇒ f (x) = ax−4−bx−2+cosx ... (i)
Differentiating (i) with respect to x, we get

d

dx

[f (x)] = a(−4)x−5−b(−2)x−3 + (- sin x) =

−4a

x5

+

2b

x3

−sinx