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[tex] \int\limits {\frac{x}{ x^{4}+ 9 } \, dx [/tex] ?

Răspuns :

facem schimbarea de variabila y=x
[tex] \int\limits { \frac{x}{(x^2)^2+9} } \, dx [/tex]

Notam [tex]x^2=t (x^2)`dx=t`dt 2xdx=dt [/tex]
[tex]xdx= \frac{dt}{2} [/tex]


[tex] \int\limits { \frac{ \frac{dt}{2} }{t^2+9} } \, dx = \int\limits { \frac{dt}{2}* \frac{1}{t^2+9} } \, dx= \frac{1}{2} \int\limits { \frac{1}{t^2+9} } \, dx = \frac{1}{2} \int\limits { \frac{1}{t^2+3^2} } \, dx =[tex] \frac{1}{2} * \frac{1}{3} arctg \frac{t}{3}+c= \frac{1}{6}arctg \frac{x^2}{3} +C [/tex] [/tex]