Question
Find the derivative of function $\frac{a}{{{x^4}}} - \frac{b}{{{x^2}}} + \cos x$ (it is to be understood that a, b, c, d, p, q, r and s are fixed non-zero constants and m and n are integers).

Answer

Here $f(x) = \frac{a}{{{x^4}}} - \frac{b}{{{x^2}}} + \cos x = a{x^-4} - b{x^{ - 2}} + \cos x$
$\therefore \;f{\text{'}}(x) = \frac{d}{{dx}}[a{x^{ - 4}} - b{x^2} + \cos x]$$= a\frac{d}{{dx}}({x^{ - 4}}) - b\frac{d}{{dx}}({x^{ - 2}}) + \frac{d}{{dx}}(\cos x)$
$ - a{x^{ - 5}} + 2b{x^{ - 3}} - \sin x = \frac{{ - 4a}}{{{x^5}}} + \frac{{2b}}{{{x^3}}} - \sin x$
$- 4a{x^{ - 5}} + 2b{x^{ - 3}} - \sin x = \frac{{ - 4a}}{{{x^5}}} + \frac{{2b}}{{{x^3}}} - \sin x$

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