y'''=cos 2x+1/x^4
[m]y`= ∫ y``dx= ∫ (\frac{1}{2}sin2x-\frac{1}{3x^3}+C_{1})dx=\frac{1}{4}(-cos2x)+\frac{1}{6x^2}+C_{1}x+C_{2}[/m]
[m]y= ∫ y`dx= ∫(\frac{1}{4}(-cos2x)+\frac{1}{6x^2}+C_{1}x+C_{2})dx=-\frac{1}{8}sin2x-\frac{1}{12x}+C_{1}\frac{x^2}{2}+C_{2}x+C_{3}[/m]