Hi. Stuck on this substitution. The integral is
(x (cos(x/3))^(-1))/(9-x^2)^(1/2)
The substitution is x=3cos(u)
After performing the substitution the answer gives
the integral.
(3cos(u) u - 3sin (u) du)/3 sin (u)
I arrived here OK, however it states that the above tidies to
-3 u cos(u) du
Stuck here on how it tidies to this
Thanks